Spring water, whether moving or stationary, is a kosher mikva.
Rain water is only a kosher mikva once it is stationary (just sitting in a pool, not flowing anywhere).
In neither case may the water enter a constructed mikva through a "kli," which includes being carried in a bucket or via pipes with bends and other places for water to collect. In the case of a pipe that may not be a kli,  consult a rabbi.