The seatbelts are bespoke to the Elan due to the internal workings of the retractor. OEM Seatbelts were manufactured by Autoliv UK. (Many thanks to a wise old gentleman called Steve who helped design the M100 seat belt).
The expensive and hard to find piece is the pillar loop (The company that manufactured them went bust about ten years ago). Only two unused ones remain in the world: both of which now reside under Steve’s desk. These are destined to be used as a pattern should Lotus ever require another batch to be manufactured.
The buckle has an alternative which is the twin stalk buckle from the Esprit. I don't know how you would fit this though.
The two most common faults are the retraction spring becoming weak and the webbing becoming worn. Total overhaul is available by FDTS based in Surrey.
They can provide new retractors which are manufactured by Autoliv Australia for approximately £25 each.
If you need a new webbing belt, because the dog ate it or you just fancy a colour change, there are 70 colours available, again approximately £25 each.
Say hello to Sue at http://www.fdts-seatbelts.co.uk/. They have direct access to AMSAFE's (formerly SafeTex, formerly Autoliv UK) parts bin (and Steve !)
If you want a new seatbelt assembly, lotus have just bought 100 units from AMSAFE.
Old part number C100U0048F New part number F100U0048F -- £76.68 + VAT