Like all builders I have a big pile of tanks and wheels under the bench, not always the same size or size needed but I have them. This week I went to a fly in and the venders had a lot of very cool stuff, both new and used. One of the guys I was with bought a big hand full of wheels for almost nothing, most of them brand new.
The bigger shows and fly ins are a great place to buy extra stuff. Club auctions can also be even better.
It's true, in this hobby a few extra bucks is just the price you pay to stay in it. I don't need any more props but every time I go into a hobby shop I usuall end up with at least one. I can't seem to ever walk out without a bag of clevises or hinges. Like Mike said, over time you don't need to keep making trips to the LHS for parts.