Ebay's a great resource for this kind of thing,...
and I have also gotten many great pieces of gear, often at great low prices. [$26 for my Fostex 3180, nyah nyah, haha]. However, try not to get too attached to one piece of auctioned gear, otherwise in the excitement of the moment, you'll get into a bidding war on something, and possibly end up paying way too much for an item. I've seen this happen many times, so don't get carried away on bidding, and be patient, because if you lose one auction, another one will come along that's to your liking.
I have many examples of this, f/i, I payed $77 for a Tascam PE-40, then the following week, I saw the PE-40 bid up to $165, in a bidding frenzy. The following week after that, I scored another mint PE-40 for $59. I'm full of stories like that, but one will suffice.
That's just an example, and your bids will vary, but the point is to not let your emotions get a hold of you, and don't get sucked into a bidding war,... unless you're sure this is a one-of-a-kind item that you must have at any price.
Beyond Ebay, sometimes your neighborhood pawn shop can have a few choice pieces of audio gear. Here in L.A., as in Hollywood, there's pro-audio gear places on every other block, and a significant number of "studio outlets" for TONS of used gear. I've been to a number of these shops, and have been no less than amazed at the amount of surplus analog audio gear that's just sitting, collecting dust. If you could see what I have seen, it would probably blow your mind, but that's Hollywood for'ya. Other major cities with media centers may have similar surplussed stuff like what I've found just scrounging in L.A.
Otherwise, for the most part, it's Ebay for me.