To address the make or buy part, I usually buy a culture of the type of ff's I want to use, and then make more cultures using the purchased ones. There are a few different variations, but I prefer the flightless/wingless variety, as well as the hydei (the bigger species of fruitfly).
As for which is the safest... I couldn't tell you for sure. I doubt it would matter too much what the ones you find have been on if you are just using them to create more cultures. My only problem with that is wildcaught ff's fly and I don't like to deal with that.