The head restraint holes are standard BL, so the D shaped restraints will fit (various colours available), and the later slimmer restraints will also fit - from 1976 on the MGB.

They are still avilable new - if buying new, specify the single pillar fitting, because some aftermarket seats have two.
http://www.mgocspares.co.uk/acatalog/MGOC_Spares_SEAT_COVERS___KITS__MGB__105.html

They should also be available on e-bay, and at Jumbles.

DaveW
'05 Red Roadster S1
'16 Yellow (Not the only) Narrow AR GDI Plus 4