There are other antennas that can be used. But the problem would be mounting them, as they require a solid mount and aren't mag mounted. As it is, you already have an antenna that can mount on the top center of the roof and work as well as a K-40 antenna - IMO. Propagation from the center roof top is best . . .
Until you can spend ~$90 for a Wilson 5000, I would work with that. A tri-mag mounted Predator 10k will set you back almost $130, and a tri-mag mounted 1/4 steel whip will cost around $70.
Keep in mind, that where an antenna is mounted means a lot to just how well it will work.
The Little Wil isn't the worst shortie antenna I've worked with.
You can do well with it!
Pictured:
Left: The SUV mount is $15, but will mount the antenna to the side of the vehicle. Which is OK; but doesn't allow for the use of the roof surface/center - which is best.
Right: The Tram tri-mag mount is $40 and comes with its own coax. THis will mount a Predator 10K and a 1/4 wave/102 inch steel whip - and quite a few other 3/8 threaded antennas. Smack dab on the middle of the roof for best results.
Both can be had from Sparky's CB Shack in Illinois, as well as other places . . .