this is fine if you trust the place that sells them to always have them in stock.
I prefer to keep a culture of my own going at all times, just in case. If you keep it in a cool room, they breed/grow slower so you'll go through fewer cultures over time. Its not like its expensive to culture flies. Plus this will make you a pro at it for when the time comes that you'll need multiple cultures going at all times for your hungry little mouths