You may be surprised to find that it isn't much more expensive to fly between Los Angeles and Las Vegas than it is to take the bus, especially if you shop around and find cheap flights, booking two weeks in advance - and it's much faster.
Southwest Airlines flies to Las Vegas from Los Angeles, Burbank, Orange County or Ontario. They often have the cheapest airfare, but don't participate in any of the online travel sites, so you have to check directly with their website, where you can also find hotel and airfare packages that make planning your trip easy.
For all other airlines, you can compare airfares to find cheap flights using Kayak.com. The airport code for Las Vegas is LAS. Los Angeles is LAX, Burbank is BUR, Orange County SNA and Ontario ONT. Jet Blue also offers flights from Long Beach (LGB).