I actually found a great resource online that I was very pleased with -- Public Lumber (
Public Lumber Company) in Detroit. Prices are great and they were able to ship a half sheet via UPS provided I gave them the go-ahead to cut it in half -- which was fine for my needs. Here's a link to their marine plywoods:
Marine Plywood. They have a decent variety and were very easy to deal with.
A 4x8 sheet of 3/4" marine teak plywood (good 2 sides) is $209.95 -- waaay less than some of the local vendors near me (one was asking $500/sheet!!). I've only bought from these guys once, so no guarantees, but I wouldn't hesitate to buy from them again.