Front + Rear inertia seatbelts

I need these for my car for it to be legal for people to sit in the back, any help/ links?

Any of the Mini suppliers can do front and rear belts, but since the Elf doesn't have mounting points in the rear, you'll need to add reinforcements to the rear bulkhead. You could take a look at a standard Mini and see what you need to do, and probably weld in some brackets, or you could get the kit from Somerford.
Rear Seat Belt Mounting Kit
I think Humph on this forum is doing or has done his Elf, maybe he'll chime in.
I put retracting front lap/shoulder belts in my car, not realizing that the Minis had the anchor point in the rear companion bin. I just put both the retractor and the anchor on the same bolt. Not the way it was designed, but it works.