I do not know specific information on Sulcata's, but In less you are 110 percent sure what kind of mushrooms they are I would buy them from the store. If they are safe and there are no pesticides/insecticides which could have compromised them then It might be okay.
I would not feed anything out of the yard. And as far as going and buying mushrooms, it really isn't necessary. Mushrooms are a great source of fiber for forest tortoises, but grazers, like Sulcatas, get their fiber from the weeds and grasses they eat. They would not find mushrooms in their native habitat. The mushrooms probably wouldn't hurt, but it just doesn't make sense to me to want to feed them. Stick with leafy greens and grasses.