There used to be a post office next to circus circus, behind the stardust on industrial rd.

Not sure if it is still open, haven't mailed anything at the post office for a few years.

I know for sure there is one on Las Vegas Blvd, downtown. If you where headed to Fremont St. you could stop by it.

201 Las Vegas Blvd S # 100
Las Vegas, NV 89101