Truthfully, though, if they already neck rein, I would just ride them that way.
Here is a super nice quality browband headstall that is very reasonably priced. I have 3 of these that I used on everything from colts in training to my finished horses and they work great.
Cowhand Quick Change 3/4" Browband Headstall | NRS - National Roper Supply - Western Wear, tack, team ropes, horse tack, team roping ropes, bits...
I agree on the snaffle suggestion, but one other thing that I feel should be mentioned is that since you don't know their tack history, they may not be used to being ridden in a snaffle.
If a horse has been ridden in a curb bit for a long time, then they may not be as responsive in a snaffle as they should. Of course, since you are new to these horses, you should really ride the first few times in a smaller, safely enclosed area like a roundpen or small arena. That way, if something happens, there is less risk to both you and the horse.