Hiya, I don\'t know the Jo Sonya product firsthand but there are many heavier-bodied mediums made for acrylic paints by W&N, Daler-Rowney, Liquitex and especially Golden.
If you only need small amounts you can make up custom mediums on the palette with talc or marble dust and many different acrylic mediums of a more fluid type (gel or liquid).
Einion