Funny you should ask - I'm going online to order the powdered form of L-Lysine tonight! My friend got hers at www.iherb.com
You can order 822 servings for about $10 (serving size is 1/4 teaspoon). Priority shipping to my house is about $5. A serving is about 435 mg which is slightly lower than what they recommend as a daily preventive maintenance dose for an averaged size adult cat.
My friend tried the capsules but found the powdered form a lot easier for her cats. She gave me a week's worth to try on my cat before I ordered it. I sprinkle it in his wet food at night and he doesn't even notice its there.