Try ColorRite (
www.colorrite.com). I got some Caspian Blue and clear coat from them to touch up my 2002. As reported in another post here, the color coat is very translucent. It took four coats to fully cover. Then two coats of clear on top. The match is very good.
The ColorRite web site does not list Triumph back to 2002, but they still have the Caspian Blue on later model Sprints, and that is the color that I ordered. I got it in rattle cans. Happy with the results, but the paint is a bit high priced. You could try contacting them directly if the color you need is not listed. I read somewhere that they were the OEM source for Triumph (saw it on the web, it must be true).
Or for another alternative, try this:
www.triumph.co.uk/uk/2046.aspx
They list the Dupont color code for Aston Green (that is your color, I presume).
Ciocc