Accessibility Testing Provider for 508 Compliance Review?

I have a number of Storyline modules for a federal client that need to go through review for accessibility issues so they can be made 508 compliant, and I'm having a heck of a time finding someone to provide this service. Doing it in house is not currently feasible for us. 

The most recent response I got when reaching out to an accessibility review and remediation vendor was that Flash is inherently noncompliant (though Articulate says Storyline’s Flash-based output is compliant with Section 508 accessibility guidelines) and Storyline's HTML5 output is not 508 compliant (same link), so they can't test it. But the client didn't complain about the format, just several fixable things that emerged from their own testing. 

How are you doing accessibility testing? Is there a vendor you can recommend to test Storyline-based courses and identify accessibility issues for us to remediate? I need to test around 20 modules each ranging in length from 25 to 60 minutes. ;) Any recommendations are welcome!