I've been surfing the net looking for just the right running martingale attachment to no avail. I'm looking for something with a snap attachment (this seems to be the challenging part), nickel/stainless hardware (not brass) flat, dark brown, and preferably adjustable. Does anyone have suggestions for me? Ideally it will also be somewhere that can ship pretty fast as well. Thanks!
Check out western or endurance tack... I used to have a snap on martingale i used to bring to xc schooling that I got from the tack vendor at an endurance race my mother was competing in. It was super easy to carry and just snap on if my horses were having "one of those days" out on the course.
You can always add the snap to the attachment. Usually they have a buckle where you would attach to a breast plate, just buy a snap from the hardware store (for probably less than a dollar) and put it on the buckle closure and voila, snap attachment!
Good luck! This may be something that is easier to find at a tack shop rather then online. They usually have a few hanging around for fairly cheap!