The undersuit is not difficult to find. The tricky part is narrowing down your choices. You can go online or you can go to a sporting goods store but for me it came down to quality vs. price. Other members w/ TK's will have different stories and you can go the route that best suits you.
I got mine at Dick's Sporting Goods at the Gateway in Salt Lake. It's a great quality suit and it's lasted me for over two years now. The problem is that it was expensive. I know you can find other undersuit variants at other places for cheaper (I think I even heard that Target or Wal-Mart had their own versions) but I have no idea if they last.
There's also the choice of winter vs. summer gear. Winter undersuits are thicker and hold in body heat better whereas summer is the opposite so you can stay cooler. I bought a winter set (unknowingly) and some of those summer troops got pretty uncomfortable. If there's a happy medium I'm not aware of it......yet.
This has only been my experience so I'm hopeful that other TK wearers will tell you theirs so you can make an informed decision. Good luck