This is not a common soil amendment to find in bulk......most typically it is offered as a bagged product through nurseries and home improvement stores. You could visit the Ostrom's Mushroom farm near Olympia (or call) to see if they have bulk retail outlets for their SMS or if you can get it from them directly. Otherwise, the attached link is the only bulk provider I am aware of but they don't have a yard - delivery only.

One of the reasons this may not be very common in bulk is that a little goes a very long way :-) SMS (mushroom compost) tends to have a rather high pH and much higher concentration of soluble salts than does other forms of compost.

Here is a link that might be useful: Joe's Landscaping Supplies

Not wanting to discourage anyone from using mushroom manure, but did you know that it often contains a high concentration of fungicides? (used to discourage rogue mushroom spores) This might be ok in flower borders, but I wouldn't put it in my veggie garden.

Thanks for the replies, I was looking for organic matter for a vegetable garden and it looks like mushroom compost isn't the best option. Guess I'll just have to do better next fall collecting leaves.