The last time I bought some, probably almost 3 years ago, I believe it was at Lowes, but Home Depot showed up in my google search for "silicone aquarium". I keep this 3 year old tube in my tank maintenance kit and used it again just last week. It's still good.
I have used many of these same tubes over the years. This is the stuff, made by DAP, though the tube I bought was blue and white and the label actually said aquarium silicone:
If you're looking for the cartridge of silicone that fits in a caulking gun for a big job, look for what Krazie said above, that it contains no mold/mildew inhibitor. If the label specifically says it's for bath, window or door, it probably contains the inhibitor chemicals. Stay away from it. It can kill a tank.
Aloha,
Mark