So basically I am going to reenact an Achaemenid Persian infantryman, which means I require the typical Iranian short sword (long dagger?), akinakes, for the impression as my side-arm. The question is, is it possible to buy these? Barring that, what sword/dagger could I purchase, that I could modify to look (and handle) like an akinakes?

Did you see this thread over at SFI:
http://www.swordforum.com/forums/showthread.p...production

Hmm, it looks a tad short... But something like that could do unless I can find a better alternative.

Neil Burridge does excellent work. See if he's got something you like or contact him for possible variations?
http://www.bronze-age-swords.com/index.htm