Google 'mature shrub suppliers' - that should give you at least 2 suppliers of mature plants. A word of caution though - I wouldn't recommend it. Young plants, like humans, have the advantage of growing strongly at both root and branch, adapting very well to new environments, and generally settling in much better. Older plants don't, and the more mature they are, the more difficult it is - they tend to resent being sat in soil after being potted, and resent even more being hacked out the ground for despatch, will need extra care in the form of regular and sufficient watering for their first 3 years, and extra liquid feeding, or you're at risk of losing them. It might be best to go for something that's somewhat bigger than what you've found, but not at, say, 6 feet yet.

Thanks very much for the advice. I'll bear that in mind.