Marine Plywood (Top Amersham HP6)

Hi
 I am looking for a smallish piece of 18mm marine plywood- no more that 1m square to fix a bathroom floor under a shower tray that has been damaged by leakage. A couple of small pieces will do as long as they are a minimum 40mm to span the floor joist.

We are in Chesham /Amersham area and can collect most times.

Thanks

Frances