Treeless saddles *I think* come in sizes for the rider as opposed to the traditional saddle sizes ime - although I am not familiar with all the brands, so could be wrong.
This place have a range of treed and treeless saddles which you can rent out to try before you buy if you want to.
If you get one off ebay do make sure it is a branded model and not one of the cheapy ones from India or somewhere - I've seen one of these stripped down and it is a complete and total deathtrap
You will get what you pay for.
If you look at Solution saddles you will be wanting a size 3 which equates to a 16.5"