From my experience, the stock grips on Yonex and Donnay racquets are the lightest replacement grips commonly available. They both weigh around 10 grams installed. Others that are between 12 and 14 grams include Prince ResiThin, Volkl DNX, Bab SkinFeel. I think there are some more light ones, but these are the ones I remember. There are several threads on this topic so you might want to run a search.
I find site-specific Google searches to be a little more effective than TT's search function. I would try: "lightest replacement grip" site:tt.tennis-warehouse.com