You can buy it from Ahead (Nero) - or for many OTHER plugins, get them from
http://neroplugins.cd-rw.org/
As MP3Pro probably has a much stronger Patent protection than MP3, you are unlikely to see an MP3Pro plugin there - it is also a low bitrate "conjouring trick", unlikely to be useful at higher bitrates (it's effect, rather than accuracy!).
There are no known free MP3Pro encoders and the format is highly patent and licence encumbered - if you have the choice, DON'T support it.