You could try Pioneer Gem Corporation - 253-833-2760
I have never ordered opal triplets from them, but
everything I have ordered I have been very pleased with. I just got
their new catalog, and they list black opal triplets at 5mm rounds
and 6mm rounds, $3.60 and $4.60 respectively. They also have more
they don't list they say.
I've gotten lovely opal solids from Rio - can't put my hands on
their catalog right away, but if the triplets are the same quality I
would be pleased. The solids are really very lovely.
Usual disclaimer, just a satisfied customer.
Beth in SC