Bearings and tape, you can buy anywhere. I usually look on Amazon (I recently got a set of bearings that came in a range of sizes - 100 each - so I just got them to keep in stock).

For derailleurs, brake levers, etc, I've had the most luck with eBay. I rode out to my local bike co-op yesterday, but unfortunately, their used parts bins were very small. I was expecting to see all kinds of discarded vintage stuff, but found nothing.

Easiest thing to do for you is to post where you live, and what you need. Odds are one of us live fairly nearby and know the local market, shops, etc. If you're a long way from sources, many of us have boxes and boxes of stuff that might help.

EBay isn't a bad place to look these days, especially as it's a buyers market out there lately, and prices of some components had gone down a bit, being that it is a depressed C&V market.....
It also seems like some components that were hard to find just a few years ago, seem to be popping up for sale more often. I think a lot of people are getting out of the fading "retro bike" craze and have started to unload some of their stash....like my younger brother, who suddenly switched over from C&V bikes to more modern bikes in his stable, just this last year....

Might try bicycleclassics.com

tape, hoods, bearings, chains, freewheels etc.