Yes, it's great for helping control cold symptoms (usually flare ups of herpes virus) in many cats. You don't need to buy any special kind, although some people buy it compounded into a gel from their vet. The cats just lick the gel off your finger.
You can buy it at most grocery stores or Walgreen/CVS type stores. I buy the tablets and use a pill crusher to powderize it... then mix it into my cats' wet food. It also comes in pill form, which people break open and mix into wet food. Some people buy it already in powder form (I think this is easier to buy online).
For most adult cats, they need 500 mg a day, and sometimes 1000 mg during a flare-up; definitely check with your vet first.
Another thing, you might try using the "search" feature on this site. There's a lot of great info about lysine in old threads.