By "Don", do you mean the Megladon from ElectronicStix?
Both 
ElectronicStix and 
EmpireMods are good sources for 
batteries. Of the two, I'd give the edge to EmpireMods, which carries the AW brand, regarded by many of us as the best.
As for type of battery, I prefer the IMR high-drain, because it provides more oomph than Li-Ion (although the mAh is less). Other suppliers of AW IMR 
batteries include 
Clouds of Vapor and 
Lighthound. Lighthound is the least expensive, but it is a flashlight supplier outside our world of vaping. Very reliable; I've ordered from it a few times (used to be the only place to get AW IMR).
As for battery size, I'd go with the largest that the mod can accommodate, i.e., 18650 for each of these models. It provides the highest mAh ... and stays at the higher end of the 3.2-4.2V charge range for longer.