Hey just a few things
A. alternators designed for road racing are a different animal than alternators designed for car audio.
B. purchasing an alternator for car audio is an extremly large purchase (very expensive) if you want one that is gonna do the job properly.
C. Go buy a deep cycle blem battery at Electro battery in St. Petersburg (23rd Ave and 32nd St.) for $40 or so and wire it in between your front battery and amps and 90% chance that it will solve your problem.
Unless you have an extreme system you should not need to purchase an upgraded alternator
But if you either do have an extreme system or just have money to burn and want to buy one google HO Alternators and Ohio Generators, these are the two top alternator companies in car audio today and you can get a quote for your vehicle from both companies on line