Computer software is great, but at sometime you actually need some hardware for it to run on. Then, for it to do something useful, you need hardware it can control.
This thing is unorganized for now, it is the bulk content stripped out of the New Items page. Also checkout Railroading, and RC Models for more domain-related projects.
My BigTrak & Robotics pages has more robot-oriented electronics info.
The plain truth is that there are many providers of components, and they all provide good service. To differentiate4 them, look for location relative to you -- which controls deliver times -- and also minimum order cost/size. The other thing is their search engines, and long term shopping lists -- so you can keep building up an order until you have enough things to get a break on shipping.
To give you an idea of a few locations of suppliers:
If looking for small quantities of parts, it is well worth looking for them on eBay. Many small electronics companies, as a side-effort, sell their stocks at reasonablle prices and cheap and fast shipping. They can make a hard to find part (or no longer manufactured) easy to acquire.