Buying Semi Mature trees?

[Andehh]

Looking for tree cover to sit just above a 6ft fence and grow to a max height of 4-5m, allowing us to (generally) plant stuff underneath it. Bushier & longer lasting the better to give us some privacy. Local garden centres are expensive & only provide small example, but we are hoping to shave off a few years of waiting for something a bit bigger.

Can anyone recommend any good ways of doing that or Recommended nationwide delivery firms, or anyone Midlands based we can visit?

Finally, any recommended trees that match the requirements above would be appreciated!