Look for clean used 6D and 30D PZM mics from Crown.
I picked up an absolutely mint 6D before Christmas for about $85 on Ebay. The 6D is the tiny one; the 30D is about the size that was licensed to Radio Shack until a few years ago. Both terminate in XLR connections. Both sound a great deal better than the Radio Shack version.
Use PrePal.com to look up what they are currently selling for used on Ebay and elsewhere. Then be patient. ;-)
If you want low cost and you're in a hurry, several vendors sell Crown's lowest priced PZM mic,
the Sound Grabber II, on Ebay and elsewhere for $60 (don't pay more than that). However, you should be aware that the Sound Grabber is intended for conference room and lecture recording and will not have the quality of the 6D or 30D when it comes to recording music.
As Darrin pointed out, you can mount the little 6D in the middle of a larger, clear piece of acrylic, drill holes in the corners of the acrylic, and hang it by wire or even heavy fishing line over a stage at an angle to the source (running the 10-foot wire from the 6D up one of the lines) for a relatively inobtrusive overhead installation. The 30 has been offered in colors other than black (I'm not sure whether the 6 has or not), so the 30 could also be used fairly inconspicuously in that way.
For long runs, I notice that David Riddle has been offering a well made ATI mic preamp / limiter on Ebay for months for around $215 plus shipping. I think he must have bought out the whole lot at a close-out price. This preamp (A/C powered) is designed to be put near the mic with levels set and left to send a line-level signal to your mixer at some distance. It also supplies the phantom power that the pro Crown PZM mics require. ATI has a good rep for their small portable preamps; I suspect this unit (though somewhat specialized) is a good value for such applications. Plus it looks like a tough little unit that would survive real-life use.
Generally, Crown doesn't let their dealers advertise their street prices, so count on calling Full Compass or another dealer to obtain the real price. Don't be discouraged by the list prices you see in catalogs.
Just to clarify, Crown owns the patent on PZM. There are other "boundary mics" available (most of them pretty sorry), but ONLY Crown makes and licenses PZM.
Best wishes,
Mark H.