The link above leads to Fibersteel, but I think their Sliding/side windows will not fit a replica. You might call Henry Reisner at Intermeccanica for a possible source. The Vintage side windows may fit but I would call Kirk (owner) before ordering them. Or, call John Steele at JPS, he probably knows what fits and has good prices on parts.
You can build your own custom fit Plexiglass side windows. Look for David Salvato or Gordon Nichols e-mail address via Search. Both have fabricated their own from materials available from Home Depot.