Any pvc window or door supplier should be able to sell you a roll. It does not have to be a perfect match for your old seal, as long as its the right thickness, thats the main thing. When you are installing it cut it 100mm longer, and feed it into the groove, as they do shrink after time. A bubble gasket is the easiest and best performing for both internal on the window sash ope, and the sash. If its the glazed part of the frame, its a different gasket, and you would need to bring a piece with you, again the thickness is the critical thing, profile can vary on the glazed units.
sabre