Doug, all cylinders need hydro, not just SCUBA, I’d ask the local fire department who is doing theirs because they have probably already done the research on who is cheapest/does a good job and then approach that company and ask for a volume price.
Considering hydros cost about $25 at the LDS even saving $10 per tank you are still looking at the price of an exotic dive trip to get 16 tanks done :(