Odd, so am I.......
you can rent 4 wheelers from some people on Craigslist. Probably your best bet.
the 4 wheelers you rent anywhere around vegas will not get you much excitement.
The heat is rolling in though and playing in the dunes will be done for sure with 90 degree temps this weekend.
your only hope is to drive up to Coral Pink in Utah where I am sure their are a couple rental places. Much cooler temps and 2 hour drive.
Apex has sand, not really dunes but enough to have fun and screw around on with dirt tires.
Logandale an hour north has a bit more sand, great trails and once again run dirt tires.
Armagosa is 2 hours nw and it is paddle city with tall fun dunes.
Coral Pink is 2 hours NE and it to has great riding for paddles
Dumont is 2.5 hours SW and great duning.......
^ What he said.
There's a place on Lake Mead BLVD in Henderson that rents sport quads, if you don't mind stock. From what I understand they'll throw paddles on them if you're going to Dumont or Amargosa but I think they're Geckos.. you'd be better off with stock knobbies. You have to transport the quad to those places yourself.
I haven't noticed anyone on CL offering to rent out their quads, but maybe if you contact someone selling one on CL they might be interested.
As stated above as well, the good riding weather is ending this week. It will be in the 90's this weekend. Aside from a race coming up in UT and hopefully a couple weekend trips to Duck Creek UT my three quads will be parked for the Summer probably.. I can't ride in the heat.