You'd be surprised where these turn up. I have seen used items on Craigslist and ebay. For ebay be sure to use the option to sort by nearest distance.
On Lexington craigslist is this listing for Berea, a small bakery is closing and has a 3 compartment sink for $500 --
http://lexington.craigslist.org/app/3561367752.htmlOn Louisville CL there's this for $950:
http://louisville.craigslist.org/bfs/3561930366.htmlthis is an older listing from bowling green CL which says it has a 3 compartment sink (not priced):
http://bgky.craigslist.org/for/3502223310.htmlyou need to play around with search terms, often broader is best so you can see how people are describing things. so "stainless sink" first... then maybe narrow down with "compartment". I've also seen "bowl" used (as in: stainless 3 bowl sink)
ebay itself has a category for "restaurant & catering" here:
http://www.ebay.com/sch/Restaurant-Cate ... 874/i.html