I don't think anybody manufactures laminated windscreens for our P6s anymore. They're like rocking horse ####!
Try eBay (somebody in the UK is advertising a full set of green Sundym glass at the moment), windscreens are offered now and then by people breaking old rustheaps! Finding glass for reasonable money however is not always easy! They're rare, and of course getting rarer... Breakers Yards might be a good bet (I live abroad where supplies are even scarcer, and the salvage yards haven't got any old Rovers in them out here :-( ).
Glass is of course more difficult to package up and transport than other parts, but most car glass suppliers will provide big cardboard boxes and styrofoam padding, as their new glass is delivered well packed. You might have to ask in advance and get them to save the stuff, mostly it gets disposed of. All you have to do is find a seller who is willing to get hold of some and a parcel service prepared to transport fragile glass! Good luck.