I'll assume it's a two piece, hinged bi-fold, smoked plexiglass style.
Personally I've always wanted to replace mine with a louvered teak bi- fold but wasn't sure how the hatch cover would work.
That stated, I'm guessing you'll need to find a plexiglass shop you cut you two panels. Then have them drill the hinge mounting holes. Of course if you have the right tools you could drill the holes yourself. Either way, i don't think you'll find an "OEM" bi-fold but then, I really haven't looked.
Good luck.