I haven't found too many saddles with interchangeable trees but I know Wintec makes them for cheap. M. Toulouse makes a saddle called the "Genesis" that has an interchangeable tree and those are probably about double what you would pay on a Wintec.
Also some of the Dover Circuit saddles have RTF systems (Rotate To Fit).
In Pessoa and Ovation the have color coded still plates (Xchange System). Which I don't know too much about but I known people with those saddles and they seem happy. The only problem with those though is that they are kinda difficult to change the tree of.
The only other one I can think of is Bates which makes one similar to the interchangeable tree on the Wintec but maybe a little nicer.