They can be found in Buy Mode > Decorative > Miscellaneous for §280.
Find the 'Fancifully Finicky Fronds' recolour used in my screenshot HERE.
All files have been compressed and tested in game.
Bathtubs aren't my thing (as in, I've never tried to make one and don't know how badly I'd mess it up), but I too hope someone takes you up on it, because it sounds like a great idea!
* NixNivis wonders if @mustluvcatz makes bathtubs...