I get them from the hobby RC shops. Hobby King has amazing prices on them. Look at the 1S cells from Turnigy. The quality seems good and they seem to hold up well. However, they do ship from China so it takes a week or two to get them.
You can also try the larger IMR cells. They have much lower cell impedance, but not nearly as low as the 20C LiPos.
I've also played around with some 5C LiPos and the cell impedance on those is not nearly as low as the 20C LiPos. The 5C LiPos are probably similar to the IMR cells. They're hard to find though. The ones I tried were some factory samples I was able to get a hold of.
I'm not sure if you understand what I mean by cell impedance, but in effect, it just means how much the voltage drops when you load them. For most electronics, cell impedance is not an issue, but at the higher currents we use for an e-cig mod, it becomes significant. The RC guys really load the crap out of them and will take the 60C ones to their maximum current output routinely, especially for the stunt planes. That's a lot of power, as much as 1000 Watts. That's well over a full horsepower!