I need a slighty longer final drive for my Sport/Street Ka box as right now i'm doing about 70mph @ 4000rpm which is not the best and also getting masses of wheelspin in the first 4 gears (all five in the wet!), I've either got a 4.25 or a 4.06 (cant remember which box I actually bought ) so I will either need a 4.06 or 3.82, just want to know where is the best/cheapest place to buy them?
I dont really want to go buying old box's and robbing them, as it's hassle and TBH I'd rather a new one as my box has only dont around 12k, although if they are silly money then that will be an option.

Thanks!