Springtails are generally sold in small cultures; usually in either a 'temperate' or 'tropical' variety based on the type of habitat you wish to use them for. I believe Josh's Frogs - Largest online herps feeders and reptile supplies store sells cultures of them, for example.
The springtails primarily feed on fungi and decaying organic matter, and will not harm your pet in any way. The culture will grow to what the environment will allow based on size and availability of nutrients. Springtails and Isopods are very handy as clean-up crews in vivaria as they inhibit mold growth and also metabolize some microbes and animal waste.