Hey nathan.
All i did to find batteries was use ebay. I will worn your that they are not all the same. Warranties are completely different and specs will be mostlikely the same but check the manufacturers descriptions. You will have the obvious variance in sizes and its dofferent in all brands. I did notice Power Agm which is a Powercrank branded battery doesnt warrant for under bonnet use and it details this in the descriptions. Ssb are brilliant and i love my 100ah unit i have fitted atm. Fullriver are the icing on the cake, one of the best and will last, but procey $350+ for 120ah. Ritar are highly used aswell in campers and motor homes and are respected highly by these manufacturers, also up in the price at $300+. Discover are a newish brand to the game and are reasonable priced at $260 fpr 120ah. Kickass by Australia Direct which i have iust purchased which i will be testing over christmas at $249 for 120ah.
I know i have priced for 120ah batteries but you prpbably get the idea. Any partivular reason you are replacing it if its proven trouble free? They should have a service lofe of 8yrs if maintained properly.
Rusty