I also make my own. Trick is to grease your counter well with shortening. Make a well of icing sugar on top of greased counter, then plop your melted marshmallow mixture into it. Grease your hands well, and knead away. I also use grocery store shortening. I confess I can’t make Ivory coloured fondant. Tried & failed miserably. I like Satin Ice, but in Canada, where I am, it is $$$.
I have a very large decorator supply store 45 min from me that make their own ivory fondant. I buy a large bucket cause it is made with white chocolate, which means I can freeze it. :0)