I'm pretty sure you won't find one in the States. I looked into getting a portable one for use when I went out to eat and such and found that they can be bought from companies in England. (This was in 2004.)
It only took a few seconds to find the links on Google.
They ran between 200 and 600 bucks IIRC and there were portable models about the size of a cell phone and table top models as well which weren't much bigger.
The smaller the package and larger the range of jamming translated into more cost.
It would be handy, if the plane you were on, got hijacked.