Try www.pricewatch.com, motherboards, PCI-express. Great place to find pc parts. Here are a few part #'s from there (no personal experience with the mobos, sorry)
many many more available ...
Have you searched froogle.google.com instead of google's web search? May be useful as well.
I'd just check the manufacturers' websites. Such as ASUS', Intel's and Gigabyte's. Look for the LGA775 socket motherboards (aren't those the only ones with PCI-e?). It's intel's 915, 925 chipsets for Intel cpus
ive seen some prices on these pci express motherboards and they have been over 200 dollars. ones ive seen have been 250 on up. at least right now i would think its gonna be hard to find a pci express motherboard under 200 dollars.