I think the only option for a rectangular one is to buy a genuine fender jag case. I was looking for one about a year ago and that was my conclusion - you can try ebay or somewhere, or guitarpartsresource might stock them, but that means shipping so will take a little while. The other option is to find your nearest fender dealer and see if they can get you one - Regent Sounds in london told me I'd be looking at about 200 quid though so I didn't bother!
