In the UK your best best is to check with ProNuptia and Moss Bros and see if they have any ex-rental jackets in your size in that price range. That way you can try them on to make sure they fit.

Many of the "inexpensive" Prince Charlie coatees advertised on line are very cheaply made in China, from inferior quality cloth. Fit is also an issue as many Chinese clothing factories are less than precise in meeting the specs laid down by their European customers.