Hey, love the avatar.
Take a look at our Biotope forum. You'll learn quite a bit about collecting plants from rivers and ditches. I think our forum moderator, Spituch, also wrote an article on the subject. Check it out in our Article's section.
As for plants in Jersey, look in the Aquascaping forum. I think Jay Luto and ryuken168 have discussed local shops. If you do find a good shop, please be sure to upload an article into the LFS section so that others find out about it.
Lastly, many of use resort to purchasing plants online. Most of the time you get a good price and decent plants. Either go to Aquabid or to Aquarium Garden. If you use the later, please use the link at the bottom of the Index page and support this site at the same time.
Good luck and let us know how it goes.