There aren't any syn free ones, but found these for you
Cadbury Light Mousse, Chocolate, ½ Fat, 4 pack, chilled 55g pot
3 Syns on Extra Easy Original 3 Syns Green 3 Syns
Asda Good for You, Chocolate Mousse, chilled 60g pot
3½ Syns on Extra Easy Original 3½ Syns Green 3½ Syns
Tesco Light Choices, Chocolate Mousse, 6 pack, chilled 62.5g pot
4 Syns on Extra Easy Original 4 Syns Green 4 Syns
Weight Watchers Belgian Chocolate & Vanilla Mousse, twin pack, chilled 80g pot
4 Syns on Extra Easy Original 4 Syns Green 4 Syns