"no matter how much you work your monitor, the printed piece will always seem darker and less saturated."

That's simply not true.  Printed images can be darker, lighter, more saturated, less saturated etc.  It depends on your monitor and printer color profile settings.

"Does anyone know where I could find a free single professional stock photo that would be accurate for testing a new printer?"

Google "color calibration" and you should be able to find a color chart you can use.

"I've had difficulty in the past printing accurately (the printed image is often darker than what's on the screen). "

You should be able to make adjustments in your printer software.

The first thing you need to do is make sure your monitor is properly calibrated. The best way is to use a spyder to calibrate your monitor and printer so the color profiles match, though I've never actually tried to do that with a television. I can make prints that most people would have trouble distinguishing the difference of colors and saturation between print and monitor. They will never be exact, but you can get pretty darn close.

Coldrake