I'm not sure you can lease a horse at a show. If you want to lease a horse, so owners will. you could look in classifieds in a paper or on a announcement board of a tack shop. But if you want to lease a horse, usually you can't for only one day. Or if you mean for a trail ride or a lesson, you would want to go the a riding stable.
There are many good horse websites, but since it is against site rules to list a URL in the answer, it would be best to use Google to find them.
There are numerous websites offering lease management or property management software solutions. "Pro Lease Software" would be a good search term for Google, as well as "Property management software", and "Lease Eagle".
One can find information about lease software in many different places. Some very good websites about it are EPS, Wikipedia, Capterra and Visual Lease.
After checking the website I could not find any information on whether or not you are required to bring your own horse for lessons or not. I would suspect however that you would need your own horse or at least a lease horse for her lessons.
If one is looking to get out of their car lease, they may trade it back to the dealer for another car. There are websites such as Swap A Lease or Lease Trader that may find someone interested in taking it over. It is possible to post on eBay Motors.
I wouldn't use these websites because most leases are written in lawyer talk and if you don't understand what it states then you might find yourself on the short end later. I would hire a cheap lawyer to draw up the lease for you.
There are many car lease and car loan payment calculators available online. Often, dealers make these available on their websites however if they do not, you can find one on www.carpaymentcalculator.net/
Online Lease Payment Calculators can be found on multiple car and insurance websites. For example, car sales websites will have a calculator to determine monthly payments and overall interest on a car for sale.
There are several websites that contain the information that you are looking for, for example bankrate.com and leaseguide.com. However there are several risks involved. When you take over a lease, you should have the vehicle fully inspected to be sure that there is not excessive wear and tear that you would be responsible for. Further you need to fully review the lease contract to be sure of the lease rate, warranty, and mileage allowance.
Some websites that I used to find a perfect horse was pennswoods.net.
Call your lease company and ask them for your payoff (or buyout).