The Target near me has them in stock from time to time. For projects, try HP thin clients on eBay.
An HP Thin Client might be nicer in some ways (x86 instead of ARM, say) but the lack of GPIO wouldn't make them a replacement for some RPi projects (and I haven't ever tried to install Debian or Ubuntu on one, not sure how unusual the hardware is).
It's easy to hack a GPIO together using the USB interface or I2C interface that's inside every HDMI connector.