Although you may be able to find datasets of 3D modeled trees and/or boulders, a problem could arise if you want to bring actual examples into your lab (:-). One advantage of mugs and staplers is that they are compact, light weight, etc. So, you might want to consider working with smaller plants and/or rocks, instead.
On a vaguely related note, some years ago I took a blind friend to the San Diego Botanic Garden (Encinitas). We examined and discussed a number of plants, trying to find structural and morphological commonalities that would translate between sight and touch.