You have a few options. I used Chamonix Express. It costs only about thirty euro each way. They will wait for you at the airport and drop you to your accommodation. Its a great service. If money or time isn't too much of an problem I would really recommend taking the train via Martigny. It is a stunning trip. Either way, enjoy.
Yeah, as Mallowman above says, there are a lot of minibus companies that'll do the job for about 30 euros each way. Personally I use Alpybus but mostly 'cos it's a cute name and they've never let me down