Order from Austin Homebrew. They sell all of their kits with an optional 1% ABV boost that's basically some concentrated wort that won't color/flavor the original recipe or throw anything out of wack.
With very few exceptions, you don't want to just add ABV to a recipe. Any good recipe balances the malt backbone, hops bitterness, and alcohol content. Throw the alcohol content too much out of balance, and all you are going to taste is alcohol burn. It takes certain formulations of malt backbone and hops bitterness to mask higher alcohol content, hence styles like Belgians (trippels, quads), Imperial styles, barleywines, etc, but the original recipe is formulated for high ABV.
So basically, follow the recipe and don't go bumping ABV willy-nilly. If you want higher ABV, find styles and recipes that start with that ABV and everything will be in balance and drinkable.
Good luck!