I've never seen mushroom manure (compost) for sale around here. I don't think there are any mushroom growers around here.

I get shellfish compost from DuBois in Arundel, their delivery fee is very reasonable if you aren't too far from there. It is similiar to Winterwood Farm Compost of Lyman, only far less per yard.

Harris Farm in Dayton used to give away for a nominal fee some very nice composted cow manure, but when we called last year they weren't selling it, at least for the time being.