You become a dispensary supplier/caretaker by establishing a good relationship with the dispensaries around you and by offering good quality buds/clones/edibles at fair prices. A lot of it has to do with salesmanship, but if your bud is whack, then forget about it. The most important thing is being careful and cautious with how you approach dispensaries, even if you've taken every single step to be certified to grow MM.

Dispensary owners don't buy from people they don't know or trust. Respect is a major part in caretaking.