They're supposed to be really simple to make, since they're just a set of IR LEDs. I don't know how expensive those are, but a wireless sensor bar would be pretty easy to make if you're adventurous.

I think I was just in Englewood last weekend, though (I can never figure out what Dayton suburb I'm in when I go there) and no store had any Wii accessories at all, save for a single used Wii controller sitting on a shelf behind the counter at a Funcoland. So online through the Nintendo store is likely your best bet if you just want to buy one.