anyone know any website where i can get the rear bench seat for my 71 chevelle

I would try throwing a "wanted" ad in the classified section of this site. Others may even reply to your post here if they have one.

If you find one you might want to verify width before purchase. 68-69 uses a wider rear seat than a 70-72 that has the plastic rear side panels with upper filler. Convertibles are narrower yet.