Cox & turner.
I have used these a few times over the years , they have a massive stock of rings for just about anything.
I recently re- bored my B31, and bought a JP piston set for it, and taking the advice on here, i discarded the jp rings and bought a set of nos wellworthy milemaster rings on e-bay for it. Very nice i thought, especially as i worked for wellworthys when i left school....
However, when i received them i realised that the ring grooves on the jp piston were a lot deeper and so i had to source some rings with a greater radial depth to suit!
Kit is the ring man at cox & turner and knows his stuff!
They're in somerset, but i believe they will send to anywhere in the world.
www.coxandturner.co.uk