My boyfriend is playing the doumbek in my next belly dance show and we need to find him a costume (black pant, shirt and vest à la Mohamed Shahin), any suggestions on where we can look online?

Did you try this search on Google?
"ethnic clothing" armenian OR persian OR arabic

Neesh, I can never remember what city you're in. I've noticed lots of stores have what I call 'crossover wear' in certain ethnic sections. Find out where in your city either the Turkish or Arab community is and you'll find a shirt for him, quick. Sometimes even in the Greek stores they'll be selling galabiyas.