Pilot Parallel are good starter pens (in fact pretty good full stop) since the nibs are created with two parallel plates, rather than a more recognisable nib, I would imagine they would suit a lefty as much as a righty.
http://www.studioartshop.com/acatalog/info_672_1.html

It seems Rotring have dropped their lefthanded range, but there maybe something on ebay or in a dusty stockroom somewhere, the righthanded versions are lovely to use but a little sterile in the quality of their line, depends what you want from your calligraphy.
http://www.blotspens.co.uk/acatalog/Calligraphy_Rotring_Art_Pens_22.html

Or there are specialist lefthanded sites – I don’t know about the quality of instruction and pens but they are written with the lefthanded in mind.
http://www.anythingleft-handed.co.uk/acatalog/master_class_calligraphy_s...

http://www.anythingleft-handed.co.uk/acatalog/left_hand_calligraphy.html

http://www.clas.co.uk/lefthanders.html

By the way has anyone used a Yoropen? They look like fun, I am sure I saw a similar product designed for a competition by Seymour Powell.
http://www.yoropen.com/

Tim