Anyone know anywhere to purchase marine grade plywood for adding a casting deck to the rear of my boat in the Northern Virginia area (fairfax county)?

You don't really need marine grade.  Just buy the standard plywood and seal it well with multiple coats of Thompson's Water Seal or something similar.

I would tend to stay away from using regular ply and thompson's water seal.  If you cannot find marine grade ply then you an substitue regular pressure treated ply for it.  Make sure the ply is dry before you use it and if you are worried about water seal it with some fiberglass mat and some resin.  Make sure it is dry before you seal it!  Thompson's waterseal is a joke, don't waste you money.

Now lets throw another opinion in. I wouldn't use pressure treated or marine grade. Even after it dries, resins will usually not penetrate and cause them seperate from pressure treated. Marine grade is a very expensive over kill just to build casting decks. The main difference in marine grade and regular plywood with external glue is the marine grade does not have air pockets left by knot holes to trap/hold water where regular plywood does. It's not like your going to have your decks constanly submerged in water. Even the boat manufactors don't use marine grade for decks or floors. Get you some B/C and seal the bottom side with resin just before you lay it and the top side with a layer of resined mat. It will outlast the rest of the boat.

If set on AB marine grade, check with some local sign companies. They use a 1/2" five ply that's about the same as marine grade, it's just going to cost about $100 or more per sheet but not the paper coated stuff.

Use exterior grade plywood with no knotholes. The glue used for exterior grade is the same as used for marine grade. Get it at Lowe's or Home Depot. Mix fiberglass resin, then dilute it with acetone to a thin almost watery consistancy and apply 3 coats to all surfaces and edges. After installing, apply full strength fiberglass resin and metting on surface of deck. After drying apply outdoor carpeting with glue specific to outdoor carpet. Use a roller to smooth out carpet as you stretch it out and roll down. Your project will turn out great.     Happy fishing.

~B.A.S.S.~N.A.F.C.~BoatU.S.~