Not sure if it's been mentioned anywhere...
The obvious way to get money tree is the 5000 point reward. If you try to harvest the "fruit" itself, you can't. You can only sell it. It also does not seem to count for collections.
So, to try force gatherable "money fruit", I had the idea to graft money tree on one of the "normal" trees in my garden.
BAZINGA!
Harvesting that gives the actual money fruit which counted for the collection and which you can either drag-to-sell or plant - thus getting another money tree.
Enjoy limitless riches!