MDO (medium density overlay) plywood. It's a plywood made with waterproof glues and a special paper coating designed to take paint. It's often used for signs.

You aren't likely to find it at the local home center, you'll need a specialty wood supplier.

would a lumber yard have it? do you need to cover with a sealer?

The availability of MDO depends the lumberyard, call around. As for "sealing" it, it's designed to be painted and its surface looks a bit like cardboard. Paint would be your best option for finish longevity. I suppose you could use spar varnish but I'd save that for real wood.

If you're looking for something "nicer" there is marine grade plywood as well.

As an aside, don't confuse MDO with MDF (medium density fiberboard), they are two entirely different products.

We make signs out of the MDO "sign board" They last many years in the northern california sun with the right primer and paint. Make sure to paint the edges real well. I recomment pre-painting everything before install. Then give it another coat after.