A question that often comes up on the Due South forum is which DVD sets available on the market offer the best options and best image quality. I thought I'd share a few links if you are looking for the series on DVD yourself.
One very affordable set is this one produced by Echo Bridge Home Entertainment:
The image quality is rather poor, but if you are not fussy about this, it might be your best bet. English is the only language available here.
The second option, which offers good image quality and both English and French language tracks plus subtitles is this collection:
The screen caps found on this wiki have been made using the latter and the Season 4 set offers a special feature, that of Paul Gross' commentary on the Call of the Wild finale episodes.
Hope it helps!