Look in shops that have pond stuff. You want either hornwort (best of all) or either Cabomba or Elodea (either will do). Most pond places sell these as "oxyegnating plants". Hornwort (Ceratophyllum demersum) is the best, and it grows incredibly quickly, even under fairly poor light. It is also very good at inhibiting the growth of algae.
I had the same problem (in Southampton!) until I realised that lots of plants will float around in the tank quite happily totally unmindful of the fact that they were not labelled floating in the shop. Shouldn't be a problem to get hold of cabomba or Java fern (which actually floats quite well); if you want something more specialised, you might want to try ordering online from Greenline.
I noticed when i went to Aquajardin in Allington Lane , that they had Riccia in little bags.
I did actually buy some 2 months ago but it became a floating bed of Algae, so i got rid of it.
They also have a very good selection of ordinary Aquatic plants . From Tropica i think!.
hiya.i pinched some of the floating water hyacinth from my pond.it looks great in my tank.nice and cheap to buy aswell.only cost me 50pence at the school fete.