I can recommend
SeatbeltPros.com, though my need was a bit different than yours.
Our rig came from the factory with 12 belted positions: driver, passenger, 3 on each of two sofas and 4 on the dinette. Makes for lots of seating options for the kids on long trips.
Anyway, I also installed three belts across our bed so that when the kids were sleeping back there they would not roll out of bed should I have to brake hard.
SeatbeltPros.com has all of the hardware you need, plus a wide variety of colors and lengths. Measure where you'll be installing them. You'll probably need the longest belts they have available, but measure first to be sure.
As others have said, it's very unlikely to find anyone willing to do the installation due to liability concerns.
As someone else pointed out, regardless of the laws, I think it makes good sense to have everyone belted while travelling down the road. Some people state that belts you install "don't meet federal safety laws blah blah blah". I view seatbelts in a motorhome like seatbelts in an airplane. They help keep you planted for unexpected movements, but if you're in a major crash where the very structure of the vehicle is failing, it probably won't matter much.
Good Luck,
~Rick
2005 Georgie Boy Cruise Master 3625 DS on a Workhorse W-22
Rick, Gail, 1 girl (27-Angel since 2008), 1 girl (22), 2 boys (23 & 20).
2001 Honda Odyssey, Demco Aluminator tow bar & tow plate, SMI Silent Partner brake controller.