I was lucky as our hunt secretary had the fabric to give to people who had colors. One of the other members is a fabulous seamstress and she sewed it on my collar. This was a hunt coat though so I decided I wanted a Frock coat.
I ordered mine with a nice tattersall lining and the colors with matching vest from Alexander James.
Horse Country in Warrenton, VA has lovely, good quality, but often pricey, coats vests, etc. I know Town and Country Tack Shop here in Tallahassee also has Hunt apparrel at much less cost.......plus there are various places on line.
If you tell them what color you are looking for Alexander James will send you fabric swatches. I did that for the robins egg blue for my Collar and Vest. They made a beautiful vest for me and it was half the price of Horse Country.
Another good source is Bookster. One of our other members got a Red Frock coat made by them. They have beautiful tweed fabrics. They take about a month longer than Alexander James although I think they might be a little cheaper. They also do custom waistcoats.