usually memory locks come with the stands. if you don't know what i'm talking about, a memory lock is like a metal ring that goes around the tube and can be tightened with a drum key. it serves two purposes. the main purpose is to lock the tube in place once you get it where you want it so it's always at that height, at least until you loosen it and reset it. the other purpose is to give you some extra protection against the tube sliding down on its own. the tube shouldn't slide down even without the memory lock so that's something else you could look into. i'd check the wingnut screw on the outer tube. it might be stripped or broken in some other way.
as far as finding a memory lock goes, i really don't know how to go about that, other than to conduct an internet search for aftermarket locks. your local drum store or music store might be able to help with that too.