Try Ebay. Its well worth the money to have a proper hacking jacket than just any old suit jacket. They are properly designed for riding, and they are more fitted than suit jackets. I know at Ag shows around my area, it is very competitive and proper turn out is always demanded.
You can find some really nice quality second hand ones on Ebay, but make sure you know exactly what size you are. My Dublin jacket is an 8, but my Windsor one is a 10. I way prefer my Windsor over my Dublin and I would really recommend you get one of them.