Josh's Frogs has a couple of types. Also, most dart frog keepers have them and there might be some in your area who would give or sell you a culture. I wouldn't spend more than $8 for a producing culture in a 32-oz culturing contaner. The melanogasters species are smaller than the hydei, but I have found that the hydei produce more flies per culture. However, the hydei are also climbers and go straight to the top of the enclosure. They are also escape artists. Both species can get through standard window screen, but the hydei have a more difficult time doing so.
They're easy and inexpensive to culture and Josh's has the media and other supplies needed. It probably costs about 50 cents per culture and all you do is add hot water to the powdered media, let it cool, add about 50 to 100 flies from a producing culture and put in on the shelf. It'll start producing in 2-3 weeks and will continue producing for a month or more.
snailtail.com is THE place for FF, I love their cultures and I have never received a culture from them with mites. Unfortunately I have received cultures from a few reputable places with mites Also the Snailtail media doesn't tend to get liquidy at the end which is a huge plus.