The type of adhesive you want to use will depend on what you are trying to mount. If its a small frag then super glue works best. If it's a larger colony or something more substantial, epoxy works better. The side affect to epoxy is that it will make your skimmer go nuts for a day or so.
Cyanoacrylate Glue is the type of super glue you want to use. If you read the back of the packing it should state what type of glue is within the container. They sell it almost anywhere and in varying consistencies. LFSs like Barrier carry (1 oz., 2 oz. sizes) which range from $7.99 - $9.99. As Reef Setup mentioned most LFSs will carry IC Gel.
Loctite makes this glue as well, but I have found it to be slightly more expensive. This can be found practically everywhere: HD, Fred Meyer, QFC..... u name it. I hope this helps.