I did not say cheap I said inexpensive. She has been borrowing them now at the shoots but needs to practice at home. Again she is 13... they are fickle. I was looking for some suggestions on some possible websites that people might sell used cowboy mounted shooting items including maybe guns.
IMO, cheap and inexpensive are the same thing in this situation.
Ruger Vaquero is the best way to go if cost is a consideration. These are very well made and popular in the mounted shooting world. This means that the quality of a used one is likely good, they will be relatively easy to find and will tend to hold their resale value. If you niece stays with it she has a good set of guns. If she gets out of it, you will likely get back what you spent (or more) on a pair of used Rugers.
In my experience, Uberti clones are not as well made, nor do they hold their value as well.
I am pretty sure you can find a used Ruger Vaquero for around $500 each. Less expensive alternatives will not be much less ($400?).
A good place to look for used gear like this is the SASS (Single Action Shooting Society) website. SASS Wire Forum
They have a classified section. Because word gets around quick in the cowboy world, SASS members selling items there tend to be a bit more trustworthy than a typical internet site.
There is also gunbroker (online) but I know of some people who got burned there by folks misrepresenting items. Caveat Emptor.
Sam and Lee Helms at Hired Gun Horsemanship (google them) are very active in the mounted shooting world. They are in North Carolina. It is worth a shot to contact them to see if they know of anyone selling some used iron.
If your niece is getting involved in mounted shooting, I am guessing there is a club nearby that you would be riding with. Your local mounted shooting club (or SASS club) can be a great resource in helping you find all kinds of used gear. If your local club can't help, reach out to other active clubs. Mounted Shooters are a friendly bunch and always willing to help new folks get into the game.