There are plenty of places around MIT that are important to be aware of then looking for parts for a prototype. Prototypers are often on a tight deadline, and often times waiting for shipping from anywhere (even Amazon!) could mean missing an important milestone, so keep these places in mind when looking to purchase things.
True Value: Economy Hardware is the closest hardware store to campus, so it's a great spot for picking up a quick can of paint or a certain type of glue. It's half hardware store and half home goods/kitchen appliances, so it can also be a good spot for grabbing "found objects" to use in your prototypes.
Pill Hardware is a full fledged hardware store a bit farther in central. Carrying Ace Hardware brand tools, Pills also has a full set of fasteners and a paint mixing section. If you can't find it at Economy, Pills is a natural next stop.
If you can't get it at a local hardware, a big chain home improvement store is the next place to check. The Boston area is ripe with Home Depots, so there are many choices and a few places to check if you are looking for an item that is typically low stock. If you're driving, the Mystic River Home Depot is closest and tends to be pretty well staffed, otherwise you can also visit the Home Depot on the Red Line at the Andrew stop.
Cambridge is lucky enough to have two art supply stores, right across the street from each other! Blick and Artist and Craftsman are great for screen printing inks and screens, specialty cutting tools, bits of foam shapes, drawing kit supplies, and more! Just a short walk into Central Square, there's no excuse not to check them out.
With the demise of Radio Shack, Microcenter is one of the few places nearby with a supply of hobbyist electronic parts like buttons, battery holders, hookup wire, Arduino's, Raspberry Pi's and more. Also a retailer for various computer and A/V supplies, Microcenter is a great place when you are in a pinch for some tech-y items.
If you are really in a pinch for something electronics related you may need to trek out to You-Do-It Electronics to browse their wider selection of hobbyist electronics and engineering parts. The main disadvantage of this place is it really is hard to get to without a car.
If you are looking for anything related to molding, casting, or Smooth-On products, look no further than Reynold's Advanced Materials. They carry a huge selection of products for prop-making, life casting, special effects and more. The staff at Reynold's are also very helpful and used to strange questions from students in the area.
Altec Plastics is a plastic supply store that has a showroom specifically for supplying laser cutter material for lasers with 18" x 32" beds at a cheaper price per square inch than McMaster-Carr. This helps you spend less while more efficiently using material. Altec recently opened an online store, but currently does not have support for tax-free accounts. However, they are happy to take orders by phone and will remove the tax charge.
Nice wood can really take a prototype to the next level in terms of finish, and Boulter Plywood is great for just that. We often grab some of their Baltic Birch marine grade plywood as a generally nice wood for routing or laser cutting, but they have other types as well. You can make the trek out there by car or give them a call and have them deliver to campus, sometimes the same day!
If you're project involves plumbing or pipes for bending and shaping, Metropolitan Pipe is a good place to know about. Not too far away from campus, taking a trip Metro Pipe can save a bunch on shipping costs for long and heavy pipes for project work.
Many products often involve an element of soft goods work, and to do that well you'll need fabrics. One spot to visit if you are in a pinch is Sewfisticated, a local fabric supply store that's not too far from campus.
The place where we get our thermoforming supply from, J Freeman Inc. will quickly deliver plastic sheets from Dorcester to keep you well stocked for troubleshooting the vacuum forming process.
Of course, if you have time and are looking for something specific, why not shop online? Here are a few online vendors to be aware of while you cruise around the web looking for parts.
One of the most important online vendors for prototypers is McMaster-Carr. If you look hard enough, you can basically find anything on McMaster, and their fast shipping helps justify the high costs. McMaster is a great place to find mechanical components such as fasteners, bearings, and bushings, as well as materials like metals and plastics in various geometries.
In addition to looking for parts for purchase, McMaster also tends to be a great place for engineering learning in general. They often have helpful descriptions of materials and metal alloys that help designers choose the best materials for their projects. They also provide a great deal of information about their parts in their online catalog, and allow customers to download CAD files for keeping their solid models up to date.
Another great resource is Misumi, which also supplies mechanical components, but also had a vast catalog of customizable parts with a fast turnaround time. As a mechanical engineer, it's helpful to know about Misumi for a variety of projects.
Another cool spot to know about is Inventables. Not quite as gigantic as the other mechanical component suppliers, Inventables has part kits, cutting bits, and project components that are great for hobbyist projects and mechanical tinkerers. They are the creators of the Carvey machine we have in the lab, and stock stuff for that machine as well as stuff for making a machine like that of your own. Like the larger vendors, they also stock some electronic components.
For more hobbyist-style electronic parts, both Adafruit and Sparkfun are good choices to browse. Check these spots out if you are looking for something like a friendly sensor or a shield for working with an Arduino. These spots are have great tutorials and example projects, making them excellent learning resources as well!