If you have a one-ear, you don't really need to worry about a throat latch(:
Actually you do.
Although most one ears do not come with throat latches some do.
Like someone said before one ears are more for show then anything else.
They are not great to use with snaffles, because the whole headstall can basically be pulled all the way through their mouth.
I use one ear headstall for everything though, they are just so much easier to put on and take off, they are also easier to fit to different horses.
They can also fall off pretty easy, like if your horse throws or shakes it head or if your horse is rubbing its head on something, you just have to watch and pay attention.
It is probably going to be next to impossible to add a throat latch to your one ear, just because there is no where for you to thread it like with a brown band but you can always try.
Most websites that sale take also sale replacement pieces, I just saw the other day a throat latch replacement piece on horseloverz.com, so you could check there.