Avoid eBay. Follow up personal contacts. Frequent surplus shops.
Most of all, don't be mercenary and don't try too hard.
I have never bought an 'Irvin' from eBay and I have in my time owned more than twenty. Many came from the widows or families of the original wearers, often with provenance. That's an advantage of living in an 'RAF town', I suppose. Some actually cost me nothing but an assurance that I would look after them and find them a 'good home' when I could no longer keep them.
I have bought a few, however. If you look at John Chapman's CD of flight (sic) jackets you will see on it two examples of RAF flying jackets. I bought one of those (from a Spitfire owner!) for considerably less than half of the price of that Irvin on eBay.