You will find the Season 1 Region 1 DVD for sale at
Amazon.com. Seasons 1-7 on Region 2 DVD are sold at
Amazon.co.uk and their other regional branches, HMV, and other reputable places in the UK, France and Germany.
Amazon.co.uk and many of their branches do deliver internationally, however ensure your DVD player can play Region 2. The same holds for Region 4 DVDs sold in Australia by
Amazon.co.au.
You may find seasons on
eBay for less money but be careful as some fakes are sold.
Even for English speakers, the
French and German DVDs may be a good option, because they also contain the original English language track in Dolby Digital 2.0.