Enzo,
I don't think they are custom at all. I would try calling Kyle over at House of Speed and see what he has to say - he used to work at HMS. Other than that, any good shop should be able to get the specs on those rings - it's only a matter of size, not a custom made piece.
You could even call JE yourself and I bet they could get you a set of the right rings. Are your cylinder walls OK? Did you get them checked? I would be curious *why* the rings failed in the first place. Until you ascertain the cause, can you really know they won't fail again?