A subpoena can issue as part of a court action. Have you filed suit against the agency? Have you made a claim for damages? The agency may very well record over the tape if you wait too long. A local attorney in that state will know how to preserve the tape in case of future action (assuming a recording was actually made).

Understand that the damages might be such that this is a small claim action and not one that you can or should use an attorney for. If your car has been damaged, submit a claim to your insurance company and let THEM deal with the damages and collecting from the police.

And, of course, if you intend to sue for a violation of your rights, be prepared to have the money up front and hope that a judge sees the search unlawful as you appear to.