Ancient spores have a 1% chance to be obtained through sifting sand. Ancient spores are used for making mycelium. This is accomplished by right clicking the spores on a Dirt Block. Note this is not the easiest way to get mycelium in many modpacks - MineFactory Reloaded adds the sludge boiler which uses Sludge to create mycelium with greater efficiency, and autonomously.
Using a sieve to get Ancient Spores...