If your car is a 420 Sovereign then I think the seat belts are the same as for the Mk2 and the S Type. I have a 420 and feel sure this is the case. I can check the part numbers from my parts catalogue if you want.
SNG Barratt do a good range of seat belts and can give you the choice between static and inertia reel types. They sell Securon belts too but you might save some money by going direct if possible. I don't know whether Securon belts would look original, if that's important to you.
There is also a company called Quick Fit (NOT the well known chain of car repair specialists) who specialise in seat belts for classics and come highly recommended from the classic car press. Try this link