I've used the FXpansion DR-008 (full upgrade from a version bundled with Sonar 2) for a good while, and it's worked out pretty well. Lots of nice kits available for download, if you're a registered customer.
However, DR-008 has not been updated in a while -- Angus has focused on BFD (fabulous acoustic drum VST) and on the new Guru virtual beatbox, and the planned Drum Nine successor to DR-008 hasn't materialized. Since I have Komplete 2, Battery arrived in my mailbox a few months back, and it seems a lot nicer than the original Battery.
That said, my next purchase will be either Stylus RMX and some Xpanders, or a new DAW. I'm a bit concerned about some reported compatibility issues with Sonar (or vice versa; Spectrasonics seems to think the problem with losing sync are on the Sonar side). Assuming those get worked out -- Stylus RMX provides ways of working with beats and grooves that are way beyond anything any other product can do, and it sounds great.
You haven't said what kinds of drums you want -- acoustic, electronic, shaped noise, whatever.
For acoustic drums ---
http://www.naturalstudio.co.uk/nskit_about.html are quite good. Bear in mind they are not exactly free:
It should be noted that these samples are NOT royalty free. If you wish to use them in such a way as you'll be making money from it (be that in any form) you must ask permission.
Reportedly, permission is not hard to get, in exchange for album credit.
- Jim