You have a pretty sizeable establishment and vaccinating is understandable for you. True that vaccines are costly and only come in "bulk" but if I could be certain there's one that will absolutely 100% work I would buy it in a heartbeat. A vaccine co-op sounds neat but in my suburban area there wouldn't be enough owners to "share" especially since the people I know always seem to be at different points of raising their flock -- some are in chick stage, others in juvenile stage, others in pullet/cockerel stage, yet others with adult or aging birds. I just resort to going to my vet for everything poultry. He used to actually work in the poultry industry with meat and layer chickens so has a lot of medical knowledge and I help him learn about the pet aspects of chicken keeping. He wanted to show me how to shove a syringe down my Silkie's throat and I politely squeezed the contents into my palm and my Silkie came running up to lap up every tiny drop with her little tongue. We're learning a lot from each other
UrbanFarmOC and I have shared vaccines. It's tough, though, since we live 20 mins away from each other. We can either split the vaccine and only mix 1/2 of it (a tricky thing to do with no scientific scales around) or hurry to do our birds and hope we make it to the other's house on time. For the Marek's, we did the first option. For Pox, we decided against sharing.