CODISCUS and Ichiban Tropical seem to have marbles fairly regularly and are of good quality. I think its about 250 or so from CODISCUS and not sure on Ichiban. CODISCUS is 100% captive bred and having seen the adults in person I can say the are solid quality rays. I'm not sure if either are to be considered "minis" or not and personally don't like this label.
To get the absolute highest quality you will need to talk to aquaman, he seems to have the best marbles (in my opinion) out there.
Codiscus def has some high quality marbles. I have a marble from him right now. Seems to be staying rather small and have had some member hint that it may be a mini. Which is a term I don't really believe in