The xenon and Malibu bulbs are both a little larger than the OEM bulb, but they use the exact same wedge base so they fit just fine. If you go to Home Depot and look for 18W Xenon wedge base bulbs you'll find exactly what you need.

P.S. The xenon and Malibu bulbs look almost exactly identical, so I had to make sure I put each bulb back in its original packaging as I was swapping them through for pics. They do have a text label printed on the glass, but otherwise they look the same.