Try this one for a layout below decks of a small merchant brig. A merchant vessel is essentially what a pirate would be using.
Lady Washington Layout
Then there's the Lynx, a privateer from the early 19th Century. There's some good drawings on their website, as well as pictures, I'm sure.
Lynx
Also check out the Amistad's website. I haven't looked too closely at the site, but they seemed to have some good pictures. I believe that the Amistad was originally built as a privateer, but even as a merchant vessel it would have been good for piracy. It was often a short distance between slave trading (especially once it became illegal) and piracy.
Amistad
Hope this helps.
Coastie