I agree that a regular suit coat or sport coat are not always suitable for conversion due to the button and pocket placement. About the only regular jackets that seem to work well are single button with patch pockets that can be turned into false pocket flaps.

I would also keep an eye on your local thrift stores. My local stores always seem to have Barathea and Tweed Argyles on the racks. The price is usually less than what you would pay a tailor for alterations

Facebook Marketplace and Craigslist always seem to have a few too.