A lot of recipes from which kits are built can be readily modified. If you look at Northern Brewer's kits, for instance, and click the "additional information" tab, you'll see a link for the brewing instructions, which include the specific malt extract, hops, and so on. For instance:
http://www.northernbrewer.com/documentation/beerkits/AmericanAmber.pdf
Now you know what's in it, you can change any of it you like, and buy the individual elements of it. My son does extract brews, and when he tasted some of my beer in which I use Maris Otter malt (I do all grain), he wanted to try it, so he took a kit recipe and swapped out the normal malt extract for Maris Otter extract. I can't recall where he gets it, doesn't matter. The point is that he now has tried something different based on an experimental idea he had.
Turns out, he really likes that extract, so it's the basis of most of the brewing he does.
You can do the same thing at MoreBeer, they have the ingredient lists under the "documents" tab and they're PDF files. Here's one that shows the extract, the steeping grains, hops, yeast, the whole bit:
https://www.morebeer.com/images/file.php?file_id=6987
I think part of this exercise is brewing enough initial recipes that you can discern differences in flavors and then you can start futzing with them. I'd suggest changing one thing at a time so you can really see the differences and then evaluate whether you like them or not.
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There are lots of extract recipes out there, many on HBT, sometimes people will specify both all-grain and extract in their recipes. It's kind of fun the first time you assemble the ingredients for a recipe yourself, brew it, and then find out you can do it.
