You can always try the wild fruit fly method, but it isn't reliable, and often attracts more ants than flies. Put out fruit, open windows, etc...I hate it...
IF you live near a college or University, preferably one with a genetics department, CALL THEM!!
Find out the number to one of the genetics offices/laboratories, call them, speak to a research student/professor/graduate student, etc. Ask if they have any spare drosophila/hydei cultures that you can have.
North Carolina State has a good genetics program, and EVERY time I have gone there, I was allowed to take WAHTEVER I WANTED fromt he garbage can. They had literally HUNDREDS of cultures in the garbage, many of them were still perfectly fine, and lasted weeks longer. I had saved 2 seperate clutches of calyptratus by doing this. It is very safe to say that nearly ALL colleges with genetics programs do research with fruit flys. Fruit flies are used in great numbers because of their fast life cycles, they are, consequently, disposed of in equally great numbers. Heck, they even gave me a half dozen fresh cultures of hydei flies because they had no present use for them. Let them know what purpose it's for, so they can be sure they don't have any contaminants in the culture(but the usually keep them pure), AND ask for flightless or wingless flies if possible. And keep in mind, people in Genetics will usually find your hobby very interesting, so if you're nice, they'll HOOK YOU UP.
Try it
Eric A