LOL I got in BIG trouble from my wife for doing this- we had a bunch of apples or something in a garbage bag that had gone off (we didn't have our composter set up yet) and flies were all coming out of it, so I brought it into my growing room= she came in and asked what the horrible stink was and I simply kicked the bag and flies swarmed out all over my collection- I was smiling and well pleased, but her, not so much
If you "get lucky" and get fruit with flies already on it, you can have flies literally overnight, it realy doesn't take long, best bet is like a banana peel and maybe some apple cores in a little jar or something (left open, natch) and you should have more flies than you'll ever need- just don't expect anyone else in your family to enjoy it is my only caveat