Any 100% pure silicon oil will work. I got my 3 ounce bottle of 50 weight (pretty thick) at Hobby Bench inside PV Mall. Any decent hobby store will carry it. You might want to get a bottle of lighter weight, and one with more viscosity, like 50 weight.
I like the heavier wieghts because they stick around for awhile, and they work great on my GBB and AEG. If you get a lighter weight as well, you can mix the 2 to the perfect combination for whatever you are putting it on.
Each bottle cost me about 3 bux.