I bought one from Clear Protector that someone posted on the Sprintusers forum:
HTC Arrive / HTC 7 Pro Extended Protector | Clear Protector
It's half price with coupon code: iwantultra
The screen section fits perfectly, as do the edges around the screen. However, it doesn't cover the edges and corners of the keyboard part of the Arrive. Overall, I'm happy with it and have had no problems after a few days use.