8’x4’ 3mm sheets of marine ply you can get at a builders merchants, B&Q ,wicks etc will also stock 3mm ply in that sheet size, but I expect won’t be marine grade...(builders merchants can usually deliver too!)
Depending on what your building it maybe better to buy A1 sheets (or similar smaller sizes) from a dedicated hobby store like Cornwall model boats, who only stock good quality, ( they have a range of woods) not that cheap chinese rubbish, as if it’s for frames then A smaller sheet is easier to manage. ( experience tell me this) it’s not always cheaper to buy a large 8x4 even decent marine grades in that size can have hollows etc, remember there primarily used for exterior build work so cuts on large sheets are small and voids don’t effect a building as much as a model.
Most ply not marine grade From hobby stores will need sealing before paint, you can use a sanding sealer but it can affect paint finishes long term... ( cracking etc) so it’s best to seal with epoxy or acrylic lacquer. Both work to seal ply effectively.