They're a special animal, and there is a loyal following of investors that buy them they even have associations like
american postal owners. com
Typically you will be dealing with the Leased Fee intrest, you need to be vey careful the lease terms many are gross and older ones are generally ten year terms with five year options.
Market rent is determined by looking at the alternitve usses for the building, be it retail or office, depending on the location and H&B use, rarely is a Post Office the H&B Use conclusion.
I haven't done one in a few years, but there is a lot of data out there, it just hard to uncover, often times the local post master can be a good source of info.
The typical data sources like Loopnet can help you find brokers in your region that may be handling some POs that can help with cap rates, which tend to be toward the lower end, given the credit strength of the tenant.