They are in most rivers and canals, you need a licence to catch them, but very easy to obtain.
The environment agency like you to catch as many as possible as they are not native to our country came from over paid over sexed and over here USA were else, I applied for a licence for my area and was told that the river Test and the Itchen and totally free of them so it was a non starter for me and I have already bought the pot, they are only cheap on e-bay if your interested