Winamp Pro links are available at the main download page
. Link(s) for WA Pro are also included in the Winamp 5.xx
threads. Note the "Already own Winamp Pro and need to upgrade?" link, a couple lines below the download buttons.
Basically the only difference between Winamp Full and Winamp Pro, it that the Pro installer will ask for the reg key during install. Essentially you can upgrade a Pro version with the Full package/installer.