You can either get "fish safe" silicone from a fish store. Or you can get it from a hardware store. The only one I know for sure is safe is GE Silicone I. Do not get II or III because they contain anti-mildew chemicals. Also make sure you get clear because silicone does come in white.
There are a lot of silicone products. Home Depot, Wal-Mart, etc have them and the right one will be clear and say "for aquariums" on the label. Same stuff as the lfs, 1/3 the price.