Human:
I got them from this vendor on Amazon http://www.amazon.com/gp/product/B0085MXZH0/ref=oh_details_o01_s00_i00They're currently out of stock, so replacements you get may or may not be the same batch.
I got one from this vendor on Amazon:
Two others I got direct from China on eBay. I forgot where the 4th one came from. All work, but 3 are only stable to around 50cm. This is the case with any library I use or if I use a sketch without a library. So they're defective for sure, and 3 out of 4 being defective makes me question the quality of the HC-SR04 sensors.
I'd say the SRF05 is the best sensor. Works perfectly and works with only one pin. It's also easy to identify as it's the only ultrasonic sensor I know of that has 5 pins (one is not used).
Tim