The kind that is made of heavyweight cotton and instead of just a trigger clip has a proper leather end with a buckle? I've searched the internet and nothing is jumping out at me
I found mine in the tack shed, It's in perfect condition and has never been used! I have no idea where it came from, I've never seen another in a shop.
My big horse would just snap the modern ones.
I'd say ebay/tack auctions/tack stands at sales would be the best bet.
You can get them on ebay as i got mine on thier was about £7 i think thick webbing strap with leather stoppers every foot and leather buckle fitting and hand loop i love it so tough and i find the little leather markers very helpful.