Ebay: Best Source for just about every part or tool needed for prototype phase and the best part is that it doesn't require you to buy multiple units. It makes a good small business supplier.
DigiKey: If you need any electronic part, tool or widget this is your one stop shop for all things electronic you will forget about radio shack after you look through their list.
3D Systems: If you don't have a full machine shop and are willing to spend the money this company can make your prototype out of nearly any material.
Greenhouse Megastore: Personal favorite of mine contains a great selection of greenhouse supplies, coverings and hydroponic equipment.
Rio Grande: Good source for small tools and jewelry making supplies, great resource as well for model making.
Kaso Plastics: Injection molding company can help design housing or parts, they specialize in short runs plus they are located in Washington so you can save on shipping charges.
McMasters: Wallmart of parts loads of things including welding supplies, electrical, plumbing, metal stock, etc.
Make Magazine: Excellent magazine full of information on electronics basic board level design, microcontrollers, and useful DIY projects.