Sometimes the big box stores will have it, or other hardware stores. It should say "beverage tubing." Otherwise, check out your local homebrew shop or any of the big online ones. There are plenty of online sources that specialize in kegging, I'll recommend our sponsors Keg Connection or Birdman Brewing. If you are sensitive to plasticky flavors, I'll second the barrier tubing recommendation. Bevlex 200 or Accuflex Bev Seal Ultra are two popular ones. The barrier tubing usually has less resistance than standard vinyl, so you'll need more of it. I don't have experience with the Bevlex, but with the Accuflex you'll want 15-20 feet per tap.