Posted: 02/01/2016 at 12:29
Many garden centres have aquatic sections. Or you can try these people http://www.puddleplants.co.uk/ They're really helpful and their plants are really good quality and value for money. (I'm not related to them). If you need advice you can Contact Annette (see the link on their site) who will advise you.
The thing is most aquatic plants aren't sold at this time of year as they're dormant - you'll probably have to wait for the spring.