AD2 is part of each ADpak; you can't buy AD2 alone. Also, the Beats come in a .XPAK file, so you need AD2 to access them. If you have purchased SPlat, you get 3 ADpaks, 3 MIDI paks and 3 kit pieces. With SPro, you get one of each. That ADpak gets you AD2.
If you're just looking for drum beats in MIDI files, there are plenty of free MIDI file download sites on the web. I use MidiYodi to test listen AND to convert MIDI Type 0 files to Type 1 (using the Trackify command in the File drop down menu). You can then pull just the the drum track if you like it.
If you have SPlat or SPro, open the Command Center, select Addictive Drums 2 and swipe/copy the Serial number shown. Then go to the XLN website, log in to your account (or create one if you don't have one already) and paste the serial number from SONAR into the Register Product Key field and click on Register Product Key. That will give you access to your 3 (or 1) ADpaks, MIDI paks and kit pieces. Use the XLN Online Installer to install and update AD2 (and Addictive Keys if you have that). Now, you can buy whatever MIDI paks you want on the XLN site and they will be available for d/l via the online installer.