Sheels has some, but not nearly as much as Utah. Fanzz has BYU stuff, but the selection is not great. Target, Dicks, Walmart, etc. all carry a couple of BYU shirts and then an array of Utah stuff.
If someone in SL County decided to actually carry a significant amount of BYU merchandise, I suspect they'd do pretty well. Putting a BYU Store someplace centrally located like Murray would be a fantastic idea.