I know exactly what you are saying, when i had those sliders mine were stripped too. I could tighten two bolts on each side, so it wasnt that bad. I did locate new threaded inserts, but i cant remember where, it was a couple of years ago and i sold the hardtop and sliders and got a soft top.
Found these install instructions for a fiberglass slider. They show the inserts. Maybe call theme and see if they have a set to purchase?