If you're willing to put some "sweat equity" into it, you can refurbish old planes yourself. I find it to be really rewarding to return a neglected old tool to its rightful purpose, though it can be a fair amount of work. There's an article on reconditioning at the link that Kudzu posted: http://www.brasscityrecords.com/toolworks/graphics/re%20hab%20no%206t.html Be aware that he doesn't show the "real work", which is flattening the sole and reconditioning the blade.

The best sources I've found for old planes at reasonable prices are Craigslist and yard/estate sales. There's a place near me that does home and office clean-outs and I've gotten some great deals there, too. You should also check local flea markets, as many of them have dealers that specialize in old tools. If you have any pawn shops in your area, check them, too. My favorite Stanley #4 Sweatheart (circa 1930) was "rescued" from a pawn shop for $10.

IMO, Ebay is a crapshoot. It seems that most items listed sell for much more than they're actually worth, as there seems to be a lot of wannabe collectors there who don't know what they're buying and bid way too much for planes that are really only good for reconditioning and re-use. You may find a good deal now and then, but you have to put in a fair amount of time searching, waiting and bidding at the last second ("sniping") to get the best deals.

Used tool dealers are also a mixed bag. I have a couple that I check once in a while, but they seem to think that any old plane is worth a fortune. ;-)