Also check out shopgoodwill.com - it takes a while to find good searches (for this I like "vintage computer") but there are often systems and parts on there. Right now I don't see a 486 (though there often is) but I do see an Aptiva, XT clone, a couple of 386 laptops etc. You can also sometimes find parts, like if you do a search for "graphics card", though you'll have to go through the results yourself to find a good VLB card. Another good search for components is to go to the Computer Components category and search for "vintage".
Prices are all over the place, just depends if someone ends up bidding against you. Even when that happens I don't mind quite as much on there though since I figure at least the money is going to charity.