If you’re in any retrocomputing communities, people there often sell some stuff at good prices. Otherwise Facebook/Craigslist marketplace tend to be better to haggle people down. Additionally a lot of the people in these groups have connections with recycling centers and such.
Condition is going be varied, usually the seller will provide some information at to their condition but it can be a crapshoot. FWIW I had a long streak of buying working machines but the last 5 or so retrocomputers arrived not working. Keep in mind sellers will often describe things as not working, where in reality they just have no clue how to use it. As long as cosmetic damage isn’t too bad though, 8-bits are pretty simple to repair yourself.
Finally an HN discussion on something I have a decent experience in!
If you do go down this path, note that retrogames has 2 similar products, the c64 (mini) and the c64 (maxi), the maxi is probably the best one to go with since it has a working keyboard.