how do I get someone to train a 10 year old wild mare?

You might be able to find a rescue that will take the mare.

That'd be my first choice.

Find out if you're allowed to train the mare. I know I'll get thumbs down for this, but there are training methods that can work for anyone, it just takes LOTS of time (think a year) and taking it slow and being patient.

An effective and fun way is clicker training. Look up Alexandra Kurland for information. Lots of kids and adults have been able to train their own horses. They take their time, though. Don't rush at all.

Otherwise, your grandmother will have to get a trainer or surrender the horse to animal control.

The thing is, if she does not keep the mare healthy by keeping her feet trimmed and her teeth floated, it's neglect and that's a form of abuse and should never be allowed. If the mare can't be handled (feet handled, etc) and your grandmother won't pay for a trainer, then she needs to surrender the horse so the horse gets the proper care.

If she is a sweet gentle mare that you can handle and get on bareback and ride around, it may be worth your time to have a trainer take her for a month or two and ride her enough so she would be rideable.

But if she is spooky, wild, touchy, forget it. You could easily spend more money getting her trained than she would be worth and still not have a horse that would be trustworthy..

Talk the grandma into letting you find a buyer for her and send her down the road and tell Gma that she went to a good loving home.

Maybe she would put the money she would have spent on the horse toward your education and that would probably make Gpa very happy.

You could get some vids on natural horsemanship training. Just take the training nice, slow, and easy and don't expect changes to happen very fast. It may take a LONNNNNNG time to progress, but with patience, it can happen.