I've used Fromex down in the Los Angeles, CA area (well its *technically* in Signal Hill, but LA is like 12 cities all under a trenchcoat lol).
Their prices for slide film is a bit on the bigh side. They do a pretty quick turnaround, the last batch of slide film I sent them they shipped em back to me in like two days.
Here's their site:
https://fromex.com/
And their address: 2699 E. 28th Street, Unit 405, Signal Hill, CA 90755
I've also sent film to Darkslide Film Lab down in Bridgeport, Connecticut.
https://www.darkslidefilmlab.com/