I would just get the L-lysine powder (or tablets, which can be crushed into a powder) from a health food store and mix that into some wet food a couple times a day. You'll want to use about 500mg twice a day while there are symptoms, and possibly a maintenance dose of around 250-500mg/day to help lessen future incidents. Stress can also play a role in URIs, so that might be something else to try to minimize.
http://holisticat.com/uri.html