Find the new Pirate Warehouse by going to your island and clicking the "Build" button. It's a huge mallet, so you can't possibly miss it. Then, click that big and shiny "New" tab and the "Buildings" tab underneath to find the neat, not-so-little warehouse.
It gives user 20 more inventory spaces for Pete's sake! Unfortunately, this comes at the price of 10 Playfish Cash. Now, there's no need to start any argument over whether this is worth the dough, because an item like this should be considered a luxury. Good inventory management mixed with the ability to let go could easily circumvent the need for something like this. But, if you're the type who likes to have more things to look at (or hold to make some extra coins off of later), by all means get to your island and grab this--with suitable form of payment in hand, of course.