The common commercial brand is D'Artagnan. They carry both black and white truffle butter. I bought the white truffle version from Scheffler's at the SLM. I had to ask for it. Someone had to go into the storage area to get it.
I have never heard of white truffle butter..only the black truffle butter and have always bought it at Cheese Boutique. If you dont find it...is it possible to make your own?