Thanks, GT! That Terrawave looks close to what I need (I would like to see some more gain). I also found that Sparco should have some out soon (very similar specs).
If I go the home-made route (mounting three off-the-shelf antennas side-by-side), I'm concerned about the spacing because I've read that it's "best" to keep the antennas within one wavelength of each other. Since I can only get the antennas I already own spaced at 5.3 inches (no less), then I can't keep them within one wavelength of each other.
Of course, the whole "one wavelength or less" rule of thumb seems to be up for debate. I've not seen anything that indicates the best approach and would welcome any info.