There are three shops in Reno, and one in Carson City. Which one you'd choose depends a lot on your dive location at Lake Tahoe, and, the profiles you've been diving.
All of the mountain passes involve an altitude change:
Mt. Rose (access from Sand Harbor to Reno) +2700'
Spooner Summit (access to Carson City) +1000'
Brockway Summit (Incline vilage to Truckee) +1000'
Lake Tahoe to Truckee, via Tahoe City (west Lake Tahoe access to Reno) has no elevation gain and is all downhill.
Sand Harbor, a popular spot on the east shore, is about 45 minutes from Reno dive shops, and about the same from Carson City.
All of the west and north shore dive spots are best served by Reno (via Truckee). D.L. Bliss, noted for Rubicon Wall, is an hour an ten minutes from Reno, again, most go to Reno via Truckee instead of circling the Lake to Carson City.
The shops:
Are all closed Sundays.
Reno, choose either Adventure Scuba or Sierra Dive Center. Both have nitrox by partial pressure, Adventure also blends custom trimix (I'm not sure about Sierra Dive on that one). Adventure is about a minute off of the freeway, and Sierra is a bit of torturous in-city driving that can get you lost if you're not a local. Both are quality shops.
There is another shop in Reno, Tropical Penguins, that I would emphatically not reccommend. I'm not sure if they even have a compressor right now, since all their goods and equipment were confiscated a month ago.
Strictly Scuba in Carson City has nitrox by partial pressure, and is closed Sunday as well.
Hope this helps!