I have a Horse Fare girth that is the exact shade as my very expensive Hadfields girth, and it only cost $99 at Manhattan Saddlery. The leather quality and workmanship is excellent. It would match the darker Butets beautifully, and I know this because I loaned it to a friend to use with hers.
I also love Edgewood girths, although they might take a bit more effort to get it to that darker color.
I have a padded fancy stitch Aramas girth. The girth is two tone. The middle is a deep chestnut that matches my butet and the outer edges are dark that matches my aramas bridle. Looks very nice and ties it all together.
Wait until Feb 10th (Beval Sale) and buy the butet leathers and get a girth in a similar color. If they are lighter you can oil them up to the same dark color. My saddle and leathers started much lighter, but with oil they have darkened quite a bit. I know mink oil works quite well to darken leather, but use sparingly. I don't see where you are located, but if you are close to one of their stores they are very helpful, so you can just bring in your saddle and they will gladly help you.
ETA: BAC's girth suggestion is absolutely right! I have that girth and it does, indeed, match perfectly.
Last edited by showidaho; Jan. 25, 2012 at 08:37 AM.
Reason: Additional Info
I have Antares stirrup leathers and girth for mine. The leathers darkened nicely and look great on the saddle. My more plain Antares girth darkened to match, but my newer one didn't. Still looks nice though!