FRAG, a gaming group based on the east side of KC, runs some good-sized tourneys some in conjunction with conventions that include more than
40k.
There are several stores in the area. The bigger ones are Game Cafe,
Tabletop and
31st Century. I've never been to Game Cafe, but Tabletop and 31st Century are both good stores that run tourneys and have good
40k player bases. Of those two I prefer Tabletop because the game area is separate from the sales area.