There are many companies that offer warranties, I will quote you one of my experiences. I bought a TR7 ( ) many years ago from an independant. The engine went and the warranty company covered the replacement by BL. It went again and BL had to fork out.
You may have heard of the great outcry bu Porsche owners when they had the warranty pulled from under their feet. I know for a fact that many owners now have a Tesco extended warranty which covers more points than the porsche one.
Just check what is covered, like any policy its down to the details, provided it covers a fair amount of the car excluding things like brakes it will be fine.
As a rule you go back to the people you buy from who will have bought in the warranty from a third party