I found the ones I'm using in the wiring section at Home Depot. I was actually looking for some small rubber-coated clamps to use for the same purpose when I stumbled onto the zip ties with the screw holes and figured I'd give them a go first. I was pleasantly surprised at how well they worked once everything was bolted up.

For reference, they're very similar to the 7" or 9" ties that appear near the top of this Web page:
http://www.wiringproducts.com/index1.html