Adafruit.com is your Everything Store for Arduinos and accessories, project packs, and electronic accessories. They sell almost every type of Arduino out there (did you know there was more than one?) and are a great source for ideas and materials lists for separate projects. Oh! And don't forget to order your Raspberry Pi!
Flinn Scientific has long since been the go-to shop for STEM teachers. It offers more than just parts, there are how-to videos, science kits, lab safety videos, and every category is accompanied by a "Teacher Resources" section that include tips on material care, frequently asked questions, and ideas on how to integrate your project or program in to the curriculum.
The Scrap Box a great store in Ann Arbor where you can purchase inexpensive project and craft materials. Customer's buy items by the bagful and pay a flat rate depending on the size of the bag.
An oldie but a goodie! Who would have thought? Nestled behind the cell phones and computer accessories, there are scores and scores of great Maker stuff! Buzzers, wires, battery packs, small motors, and really helpful, knowledgeable staff.
That's right, it's really called Evil Mad Scientist. It's an awesome online store that carries EVERYTHING you can think of related to making, technology, and even robotics. They sell hundreds of components, accessories, and kits that make projects easier!
Flinn Scientific image citation: https://twitter.com/FlinnScientific
Scrap Box image citation: http://www.facebook.com/pages/The-Scrap-Box/34533714610
Radio Shack image citation: http://www.digitaltrends.com/mobile/radio-shack-releases-black-friday-ads/