Hi Ester,
There are many species of springtail, 6000+ known, and estimates of upto 50,000 with unknowns. There is a good chance that a species that achieves 5mm lives in Germany where you are. I've had success collecting larger ones using a small "pooter" (made out of a 35mm film canister)and a bit of patience. If you place soil and leaflitter in a tub it will be easier than crawling around on hands and knees! Judging by the speed they colonize tanks they wouldn't be too hard to culture. I'm not sure how long they take to grow to full size though.
