It's in the repos. In Synaptic search, use the term "cursor" to get a list of available themes.
Yes I've done that.
I find it strange that I have two users defined on my LXDE system, mine and my wife's. I find that the same cursor theme selected on both seems to behave differently! The Crystal one does seem to exhibit the same behavior on my login where the mouse pointer disappears when a pop-up appears. I expected all mouse themes to behave like they do on full KDE and still be there whether or not there is a popup or click-able button or link. So far I've only found one on my wife's login which works as I expect
It is very confusing for someone who basically does not like computers and uses them because' that's the way we are going now' but expects the same behavior as Windows, a mouse pointer should be there whatever it is hovered over. Certainly my full KDE system does this and I expected LXDE to be the same. However on this old laptop running a full KDE system is a no go.
I see no reason for the pointer to disappear and I'd expected all cursor themes to behave the same way.