Lowes carries aquarium grade silicone. I found it after using GE silicone I to glue my sump baffles. You can find it in the section with the superglue, epoxy, jb weld, etc...
From the discussions I've read, aquarium rated silicone should work fine for smaller tanks, or in this case an overflow. However, as RC mentioned, you may want to go with something even stronger like RTV108.