First stop should be a local lighting gallery, take one of the existing pieces with you and see if they can match or order it for you. There is a company out of New York, Angelo Brothers that will most likely have a near exact replacement. Almost all lighting galleries have access to them and should have one of their catalogs. If they cannot help you they should be able to point you in the right direction, Lowes/Home Depot do carry a limited amount of replacement glass, but it is usually not very decorative.
Greg Borr
RTS