To repair this, you could get a used buckle only, these should be plentiful at a junk yard. Or you could use the buckle off of your center front or rear seatbelt since this belt rarely gets used.
Then take the new (used) buckle and your old seat belt assembly to a upholstery shop and see if they can remove the old one and sew the new one in place.
It's just a matter of cutting the thread to remove the old buckle an sewing the new one in place. Some shops might tell you they can not do it because of liability issues due to it being a safety device but other shops might not care.
Should be a fairly inexpensive repair.