You can get just a flat, which is more like 5 lbs. I’m not sure what a Weber Smoker Kit is, but I have a Weber Smoky Mtn 18" diameter bullet-style smoker that I used to only use to smoke flats when I did brisket, because I didn’t think I could fit a whole flat & point cut. But I saw an episode of Cook’s Country that showed how to cook an entire brisket in a Weber kettle-style grill, which I also have. I did a thread almost exactly a year ago on my results copying their technique, which includes a video of the CC episode. I followed the instructions in the video closely and it turned out freakin’ amazing.
As for where to get a brisket, I would look for a specialty meat market in your area if you can’t find one at the grocery store.
As for leftovers, when you cook a whole brisket you do want to ideally plan to invite enough people to help you eat it. I don’t remember any brisket leftovers lasting long from last year! But pulled pork freezes and rethaws very well. I often will smoke two big pork shoulders and package up leftovers in several quart bags to freeze. Nothing better than wondering what to have for lunch on some random day and remembering “oh yeah, still got some frozen pulled pork and sandwich fixin’s!” I’m not as sure about brisket, but I imagine it freezes pretty well too.