The Pocket Pro never supported MOBI DRM, so it does not have a PID.
The Pocket Pro does support Secure Adobe ePub and PDF. You authorize it by connecting via USB to a PC or Mac and opening Desktop Adobe Digital Editions and selecting authorize. This copies a key onto your PocketPro that acts like a PID in some ways, but you don't need to know anything about it.
If you want to buy Kindle ebooks for the Pocket Pro, see
Adventures in discovering the K4PC PID.
If you want to buy MOBI ebooks for the Pocket Pro, the usual approach is to use your PC's PID and then mobdedrm.
Both the AZW and MOBI procedures involve removing DRM, which is probably legal most places for your own personal use on ebooks you bought. Many ebooks are now available in Adobe ePub, so that is often the simplest format to buy.