The last 2 times I needed a radiator caps for a Japanese cars, I got one from Mexico and one from South Korea. Getting one from Korea made sense to me, because it seems that Hyundai and Kia have the same radiator caps as Toyota and Honda.
It seems like practically every Japanese car has the same shape radiator cap, but different pressures. Perhaps some tuner company makes a premium radiator cap with the correct pressure. TRD, GReddy, HKS, maybe?