Ebay. They are all US releases, but quite readily available, and fairly cheap.

Below are the actual listings I bought them from, so if you are interested, just see if they have any more in their store listings.

Animated season 1
http://cgi.ebay.com/ws/eBayISAPI.dll...m=200298740412
Animated season 2
http://cgi.ebay.com/ws/eBayISAPI.dll...m=120359962195
Cybertron series
http://cgi.ebay.com/ws/eBayISAPI.dll...m=110323722044
Energon series
http://cgi.ebay.com/ws/eBayISAPI.dll...m=200296969643
Armada series
http://cgi.ebay.com/ws/eBayISAPI.dll...m=290285057079
http://cgi.ebay.com/ws/eBayISAPI.dll...m=370135579782
RiD complete series
http://cgi.ebay.com/ws/eBayISAPI.dll...m=360123440359
http://cgi.ebay.com/ws/eBayISAPI.dll...m=400023856301