for tamiya parts...look in the front/back of the instructions and it tells you the part no.s for each sprue tree, you just need to go to a model shop that sells tamiya models they can get you the sprue tree.
as for AMT Ertl parts that might be quite hard as they have been out of business for about 10 years. best bet is to try and find the same kit on ebay and buy it again. but expect to pay £20 +
Thanks Richard.....got some info from Geedubya about AMT Ertl and enquiries are pending. I'm gona run the windscreen issue by my "local" decent model shop (best one I've seen is in Glasgow - I'm in Perth!!!!) as they deal in tamiya and can do custom orders sometimes. I'll update the thread as I find out more.