You can order directly off the Mirage website or any shop with an account with them can order parts for you.
You can run any cells you like with brushless systems, due to the lower losses they do get more benifit from newer cells though. Capacity isn't really an issue these days, 2wd I come off with more than half the pack left and 4wd almost half a pack on my 9 month old 4200's.
Something to consider is the weight of the cells, the lower capacity cells are usually slightly lighter and have less punch, which is what you want with brushless in 2wd. In 4wd you can almost never have enough power if you're sensible so the higher capacity, more powerful cells can be an advantage
