Don't your local tack shops carry narrow English girths? Or you could always try ebay
. And I really like the merchandise of horze.com - have bought several items from them.
But what a nice saddle you have! Very beautiful. Is that an Aussie saddle?
This reminds me I need to buy a new girth too, eventually... I got my current one used and it will fall apart one of these days. I also need new stirrup leathers and I would love a new sidepull, not one that is composed of spare bridle parts. And a tree that grows money. Ugh...