There are only a couple of boats that go out on Lake Mead on a regular basis, and they're both private. Lake Mead Tony might be free to take you out for a dive or 2. The other guys are usually doing deco dives. Steve is right to recommend Lake Mohave, except, the wind really blows through the canyon this time of year and stirs up some white caps, which in turn churns up the water decreasing visibility. You also might want to consider a dry suit. The water will be in the high 40s to low 50s.
I regularly get fills at Gr8 Divin in Boulder City because they're the closest shop with hydrocarbon free air. Most of the other shops only have grade E air. It's not a bad shop, but my only experience is with air fills. Let me know what dates you're looking to dive. I'll be in the area in January and may be able to get a shore dive in if you're interested.