If you keep an eye on Craigslist these pop up regularly.
Also check out the book Build Your Own Home Darkroom. It has designs for many handy items, including a sink. It may be even less expensive to build than buy.
If you have access to a table saw and a place to work, you can make an 8-ft sink that's wide enough for 24-inch trays from a single 4' x 8' sheet of 3/4" ply and some strong paint.I've made three. The first was of particle board: leak at the drain melted it. The second was wide enough for 16x20 trays: then a friend gave me 20x24 trays which required the third version I will scan my drawings and email them to anyone who is interested.