Initially I've been using this site https://sdp-si.com/, which research shows is pretty good. However since needinng idler rollers for my project, and seeing that they are $15 a piece there (pretty steep for a little wheel). I've been wondering if there are other sites like https://sdp-si.com/ that are perhaps a little more geared to hobbyists especially in pricing. I get the impression those prices are for people repairing assembly lines or something.
Servocity and smallparts might be of interest.
For parts, wheel, gears etc...mcmaster carr http://www.mcmaster.com/#
Lifesavers. Thanks. I'll look into those.
I haven't actually gotten into robots and gears yet, but some of the other links I've looked at include:
I've bought stuff from Robotshop and HobbyKing (and Pololu). I'm in the US, and the stuff from robotshop and pololu came quickly. HobbyKing came slower (from china) -- evidently you can select to order from their US distribution center, but I didn't notice.
I bought some clevis parts to interface with the horn on the hobby servo that came with my Arduino kit from both HobbyKing and a local RC store, but the hole on the horn is too small. So, I presume at some point, I'm going to need to upgrade my servo.
Makerbot.com :) Or makergear.com, ultimaker.com, etc.
Once you make the admittedly large initial purchase, it's like having a small parts store in your house. Just as long as you only need plastic....