There are some places that sell mushrooms made of a sort of garden concrete with a natural-colored wash, and let me tell you, it is absolutely IMPOSSIBLE to tell them from the real thing. The sculpting and shapes and textures are perfect. Not cheap, at about $8 for the small ones. I bought just three of various sizes, and ended up spending nearly $30. But they are really effective, 'cause they are so realistic, and put an interesting focal point in hidden places.
Unfortunately, I can't tell you where to get them. I know I have seen them at the Backyard Gardener (I think that's the name) in the Houston Heights neighborhood. The main place I see them is the Mushroom Man, who has a huge garden decor booth at the bienniel antiques and junk sale up in Warrenton (near La Grange), Texas, in April and October.