Lots of spots. Near the Montenvers railway, La Praz, Grands Montets cable car, Vallorcine etc. Just have a drive up the valley and see what you fancy. Always loads of vans, motor homes etc around.
My favorite spot is on the main road up the valley, just after the MBC by the bowls. Clean water, clean(ish) toilets and within walking distance of a damn fine pin. Can be a little noisy though. By the Montenvers is also good.
I parked up by Le Gailliands for quite a long time. Good toilets with a tap under the sink for filling up water. You are also near the start of some good running trails and the crag for playing around on in the evenings.
The Best Western might also offer a Spa for those that don't live there. At least in the past it was possible to use the sauna for a fee. This might be a nicer option...