Need help with my OHP setup

I have a Buhl 121 Overhead projector with a Sharp QA-1400 which projects a nice 100 inch image on the wall. Im using fxl bulbs and the brightness is acceptable. But the overhead has a singlet lens and I find it lacks a little sharpness. Does anyone know where to get a doublet or triplet lens for this purpose? I looked on ebay but just found those crappy "blow up your tv's image" lenses. Thanks.