try.... Two Wests .co.uk or eden-greenhouses.com as they both seem to make green houses , perhaps supply them with a photo and dimensions when you make contact.
Might make contact using the Info@twowests.co.uk or ( this one is valid ) info@eden- greenhouses.com
I myself would also look around for a big garden centre that has a greenhouse franchise as they often have a fair range of bits to repair old ones.
Eden do sell extra windows according to their literature , I have their brochure in front of me as I have just purchased a new 12' x 8'4" Eden highline jobby .
I seem to think that most widths and sizes are standard ones so you may be in luck.