If a big city paper covers your area (hell, any paper in your area) check for an auction section in the Sunday edition.
Like wise any local classified papers (Pennysaver, Tradin'Post...World, Nickle something or other etc.) will have an auction section.
Check all the auctioneer ads and see which ones have web sites.
Most of them will e-mail you what's coming up.
I'm beginning to see a little trend starting in "online" and "online only" auctions and there seems to be a growing number of specialty auctioneers pertaining to food service equipment, at least in my area.
There really are a lot of great deals to be had but make sure you check out what your bidding on in person and remember to add in the buyers premium you'll probably have to pay.
Also make sure you read all of the buyer information for any caveats like inspection times and pick-up time requirements and such.
I seen a nice 6x8 walk-in with condenser and compresser go the other day on an online auction for under $300...SWEET

!
(this one had the caveat that you needed a licensed refrigeration dude (or dudette

) to recover the gas properly before removal)