Did it crack because something hit it, or just cracked while riding down the road? The mounting of a windshield has to be just right or you will stress the plexi and it will crack. The mounting bolts through the windshield are the usual culprits. They need to be mounted through rubber shock absorbing grommets and held with locking nuts that don't require tightening so tight that they flatten the grommets. (I use the nylon lined nuts that lock themselves in place as tightened.) I usually use fairly cheap (under $90) clear plexi windshields. I think I bought this type for my last bike from J.C. Whitney's on line catalog.
http://www.jcwhitney.com/autoparts/ProductBrowse?storeId=10101&Pr=p_Product.CATENTRY_ID%3A2000659&productId=2000659&catalogId=10111&N=111%2B10211%2B600010293&rlid=group_listing
