I'd check with most of the major online suppliers (Austin Homebrew, Northern Brewer, Midwest Supplies, Brewmasters Warehouse, etc.) and see what they offer for RIS extract kits. Usually they have an ingredient list so you know what goes in it. From there you could substitute some hops, try a different yeast, or whatever it is you wanted to change up.
But in all honesty, the kits are usually pretty tried and true and unless you already have a bit of experience with various ingredients you are better off dialing in the brewing process with a good recipe first, and experimenting after you're already perfecting those recipes. Otherwise you have a lot of variables in the mix that can make it hard to pin down what might be giving you beer that isn't so great.