There are several brand names out there.
I have gotten it from Walmart, Staples, even the local grocery store (in the little office supplies section)
When it is new, it's fairly sticky and can leave behind some residue. Most of the residue can be removed easily. I use a ball of the stuff and just roll it over the residue. You will also see a noticable line where you masked...this usually goes away with a clear coat.
Once the stuff has several uses and the paint has been mixed in with it, it'll be less sticky and won't leave the residue. I have a ball of the stuff (little bigger than a golf ball) that I have been using for years (over 200 builds masked with it) and it is still good. After use, just knead it back into a ball. Before masking with it, it helps to knead it, twist it, pull it. This helps to warm it up a little and soften it, making it easier to work with.