How hard is it to Find a...?

How hard is it to find a 100% Broken and totally Bombproof 14 HH Pony that can be Ridden 2-3 Times a week? We live in a Very small Area (Newcastle Upon Tyne) and a (Adult)friend wants to Loan a pony for me and her children to take lessons on. Doesn anyone know of any sites that could help us find one? They are really hard to come by.