I found some pond plants in Morrisons, quite reasonably priced (well I thought so).
They had 'bull rushes', mini looking bullrushes (forgotten the name sorry), lots of different water lillies, irises, grasses etc.
I got a Nymphaea Alba hardy water lily complete with basket, food and stones for £3.49 - could have just got the plant for £1.99. They also had some 'value' packs with a few plants in.