Stewart,
Welcome, you live in a nice part of the country. Please post some photos of your pond.
In the UK, I'd recommend
https://www.pondplants.co.uk/index.php
They're based at the southern end of the Malvern Hills, and it's a business that has grown out of a hobby. Unfortunately, they don't open their premises to visitors any more, which is sad because they're in a picturesque location. Roger, the owner is really passionate about water lillies, and knows his subject inside out. As well as lillies, they also supply all sorts of plants, and have quite a range of native plants, including deep water species.
I have lillies I bought from them over 15 years ago still going strong.