There are some commercial varieties like [link|http://www.cdw.com/shop/search/results.asp?grp=WAT|this].
You can also roll your own (like from a [link|http://airshare.org/learn/articles/features/pringles.cfm|Pringles can]) but I don't have any personal experience with this.
(NOTE: the home-made variety are typically used to strengthen signal in a single given direction to provide long-distance wireless links.)
I used to have a split-level townhome and used a commercial antenna for my AP (located in the basement) and it worked fine.