You can use any brand of servo you want as long as the wiring is the same as what JR uses. You can direct fit, Futaba, AIrtronics, Hitec, and KO with any problems.
Castle Creations (Since 2005)
Bumps & Jumps RC (One of the best racing facilities around)
ST Racing Concepts (STRC) Great aluminum upgrades for many brands.
Custom Works (Best Oval Cars around). Too many titles to list
Shout out to: Sticky Kicks, CR Graphics and Race Ready Products