Plainswalker availability?

senectus Posts: 1 Just Dropped In
I started this game a while back when it first came out in beta. then came back again recently.
What I've noticed in the forums is people talking about plainswalkers that i dont see as being available.

Whats the go with this?

Do they become on sale randomly? do their availability ebb and wane due to some schedule or just randomly?