Do you mean ones to stop the horse lowering head to graze - or to stop reefing reins? We had to get some for my daughters pony who kept reefing. I am in australia and we bought this one Home - The Perfect Grass Reins
and it has worked well. So well in fact that now we only need to put it on occasionally if she starts misbehaving or hasnt been worked in a while, as she learnt that if she put her head down, she pulled her own mouth. If you look at it the concept is really simple - just a band that velcros to the headpiece of the bridle with a loop on each side, and then two straps, one on each side of the horse, that clips on the D rings at the front of the saddle, runs through the ring on the headpiece and clips onto the bit. You can adjust the length to allow the horse a bit of room to move, but if they go further down than you want, it pulls on the bit and stops them.
I have also seen just a simple strap from front D ring to headpiece and this works well too, but I like the idea that instaed of just physically stopping the head moving down, it creates a consequence for the horse that doesnt feel nice (ie pulling its mouth).
Anyway, that's my 2 cents worth. Hope that was the kind of thing you meant