AusCitrus is definitely a good one, and definitely do international orders. There's folks in South Africa, including CRI, who seem like they ought to work, but I've had a difficult time communicating with them (I have tried.)
It seems like Citrus New Zealand was supposed to have a trusted budwood distribution program, but I've also struggled to find information about it. Have you contacted them to ask about domestic seed suppliers? (And if you do, or if you already know, I'd be very keen to know what kind of seed they have available, and whether they ship internationally! I've read some historical accounts that include information about when different rootstocks came into use in New Zealand, but don't know anything about the contemporary landscape. I assume it's probably pretty basic, unlike Australia which has had an infusion of Chinese funding and germplasm to support closer ties with China around the citrus industry — all of which is export-restricted.)