Cops often get sold garbage security products (for example gun locks in their cars)

It's harder to fuck up encryption than physical locks, though, assuming you use an established library and don't roll your own.

The hard part is getting the hardware though

If you actually expect to be able to break the encryption, rather than stealing a working unit, then you probably don't need access to police hardware. You can buy a software-defined radio (SDR) device for less than $30 which, combined with the right open-source software, should be perfectly capable receiving & decoding any digital radio format. It would take some mild technical skill, but no matter what approach you take the biggest hurdle will be trying to attack the encryption, not decoding the digital signal, and if the encryption is competent it'll probably be basically impossible.