First of all, if you're unauthorized and trying to monitor digital encryption, you can forget it. I'm fairly active in the two-way encryption world and I've never heard a verified account of a non-government entity successfully cracking digital voice encryption. And that's the whole point.
~otherwise...~
As far as I know, you'll need a transceiver capable of the same encryption scheme you're interested in hearing.
For example, Motorola Securenet is a very popular line of digital encryption products. To heard, for instance, your local sheriff department's Digital Voice Protection (DVP) traffic, you can use any Securenet capable radio, a DVP encyption module, and, of course, access to the correct encryption key and a keyloader. This equipment used to cost a fortune but prices have dropped significantly in the past, say, 3 years. Motorola Sabers are a prime example. I wouldn't pay more than $150/w encryption module.