Rob-
When we lived in the S.F. Bay Area, we had several local sources of teak and all kinds of other exotic wood building materials because of the proximity of the marine building and repair industry there.
It was also desirable and important to be able to go into the store / lumber yard (indoor) and pick out each piece of wood for grain pattern, quality, and size to avoid unnecessary waste of expensive wood.
So I'd first suggest you look to find a local source, if you can. Any local lumber company will be able to direct you to the closest alternate source if they do not stock the marine grade wood of your choice.
I've also bought some pieces of teak veneered marine grade mahogany backed plywood on ebay that were small but very usable that came as left-over cut-off stock from custom boat manufacturers.
If all else fails, try Google!
Good Luck!
Joe.