Presuming they're not available (or are too costly) at your local Chevy dealer, eBay leaps to mind...
Wherever you get them, I'm pretty sure you'll need the older-style fob to work with a '99 Corvette; at some point during the production run of the C5 (I don't know the model year) the fobs were changed, and the newer-style fob is incompatible with older C5's. I believe that the primary way to distinguish between the two styles of fobs is either the presence (older) or absence (newer) of the slider switch for activation of the passive RFA feature.
