There are a couple of shops in the town that hire biggish rigs (Kona’s seemed to be favoured as the hire bike of choice), not sure on prices however.
I’ve stayed with Bike Verbier a bunch of times and not known them to do a day package, but no harm in asking. Likewise the guy’s that run Powder & Dust may offer something. Depends on what your into, if your just looking for downhill then the bikepark has plenty to keep you interested for a good day. If you want more natural, off-piste riding then you’ll need a local.
I know someone out there guiding, so depending what day your out he may be able to help (it would need to be a Weds.)!
Hope that helps…..