sometimes I make them, sometimes purchased, depends on the type of tree. Cheap plastic foliage from junk shops can be re painted and flocked to look good and nothing like what they start as. I have some I got last week for $1 which I will repaint and use as water plants in a river, and the water will be 2 part clear resin with paint underneath to give the illusion of depth.
On my to do list is to make some pine trees out of coconut fibre, balsa wood flock and paint.
the majority of ground covering is flock (painted sawdust) which I produce myself, which is cheaper than buying it, painted foam shrubs, lichen bushes etc.
The foam is recycled packing material, which is usually an environmental problem to dispose of.