Anyhoo, I did some quick searching, and found two things.
1.) It's now being sold by ThinkGeek. Right here: http://www.thinkgeek.com/cubegoodies/toys/5ac8/
This is kind of good, but kind of not good. Why? Well, because they charge more for it (no big shock there) and they don't give any credit to the place they found it (again, not a big surprise). Oh, and they only resell a few colors, and the original place has it in many, many more colors and for a much better price.
2.) The original place that sells Thinking Putty is Crazy Aaron's Puttyworld (as far as I can tell). You can see the full line of stuff here: http://www.puttyworld.com/products.html
They sell a lot of colors, including magnetic black (it's got microscopic iron particles in it), or color-shifting "illusions", which look awesome.
Oil Slick (one of many cool colors):
Anyways, check 'em out. They're fun. And at under $10 a tin, not all that expensive.