Go to xljoinery.com you can download their brochure and look at the range of doors. I have just purchased exactly what you are after. The site has a store locator so you can find a stockist in your area.
Have you looked in your local DIY sheds? Focus? Homebase?
What about a local Bulders Merchant like Builder Centre?
You'll most likely have to trim it down anyway so get the next size up. You need to measure the tight size of the door frame and then deduct 10mm to give a 5mm edge clearance all round the door when closed.
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