A good PS2 controller thread hasn't been created on CAG for quite some time, but this is the only one mentioned/praised whenever it would come about, especially being wireless.
Amazon also has some resellers listing them pretty cheap, and there are craptons of them that have been sold cheap as a refurb through places like Midnightbox.com in the past. If you don't mind opening it up and cleaning the buttons, you'll be fine with a refurb/used, otherwise stick to new. It's actually quite easy and very effective to open it up and goo-gone out any sticking buttons, I'd imagine it kind of feels like the gamer equivalent of cleaning out a gun.
If you really want an official Sony OEM wired controller, I have a still-in-the-weird-white-bag brand new black controller that came with my PS2Slim. Never used, still has the impossible-to-put-back-just-like-this twisty-tie wrapping around the cord. $12 shipped sounds reasonable, PM me if you're interested.