Meadspeed used to sell them - was waiting list (made by Bartel Engineering in NI - run by the 1970's racer Asa Moyce from memory). He's old school and doesn't do websites/email last time I contacted them by phone).
Also, Goodbits (Andy Smith -
https://good-bits.co.uk/product/cr-750-alloy-fuel-tank/) also do them. Both are close replica's of the original - but expensive,
Depends whether you are building a 'replica CR750' or a cafe racer - in which case you have a wider cheaper choice. If CR Rep, you'll need to budget for a central alloy oil tank for £650 inc cap plus £1000 for race exhausts (and you need considerable mods from standard set up - I've been there!
Price has gone up considerably in last few years