There has been no K25 for quite awhile. It lost out when E-6 films were so much improved with the grain technology plus easy processing. Velvia 50 is the standard now by which all other slide films are judged.
Velvia is normal colour transparency film, process E-6. Kodachrome uses process K-14, which is getting harder and harder to have done. There are only a few places in the world that still do it, and it's expensive as hell. You can still buy Kodachrome 25 if you look around (B&H sells it, I'd imagine...). Really, some of the newer emulsions are probably better choices.