A while back I did a search/survey to see what was out there for that idea and I didn't find one. Large spheres sure, but for a truffle size--nothing. I did find a few places that had pre-fab'd shells but who wants to use someone else's chocolate from origin unknown.
Do you want to make your own truffle balls like the ones that are commercially available? If so, Chocolate World of Belgium has recently released a new two part magnetic mould for making them and that mould is available in North America from Tomric Systems. Contact info is 716 854 6050 or www.tomric.com (I don't think the mould is on the Tomric website yet).