I would suggest ebay.
Dont buy a very cheap one, they are a waste of time.
I had a basic one but wet sand confuses it.
You need to get one that discriminates between metals, dont buy a chinese one (some have outlandish claims).
You get very much what you payfor on them, expect to pay 100-200 for a reasonable one.
Have a look at
http://metaldetectorreviews.net/
robin....