Sourcing a HEPA filter should not be excessively tough.
If you have some "big industry" in your area (i.e. medical centers, surgical centers, computer component assembly or manufacturing, etc.), consult a large HVAC company - Heating, Ventilation, and Air Conditioning. "HVAC" is actually a good keyword to use in a search. You want a company which provides clean-room services - design, build, maintenance
The bigger companies will be familiar with the materials you need and can likely help out. You never know when they will have some random filter (or filters) left over from a job - still in the package. Or maybe a dented unit to let go for cheap.
^ These guys have a contact page. And there are at least three more companies returned in the same search results.
I would start by making a concise, well worded email detailing exactly what I'm looking for as far as filter specifications. The shorter and clearer the message, the more likely you are to get a busy worker's attention and interest. A company like this would want to get paid for any design work but if you approached them with specifications in hand, they might be able to help on the cheap.