I've only recently began ruler work. I learned about it on Amy's blog
http://www.freemotionquiltingadventures.com/
and trust me, asking for the ruler foot at a dealer will only bring back a blank look
It has a different name, convertible something.
I ordered mine for my Janome here
http://www.boersmas.com/ ask for Mr Boersma in the repair shop. It was $18.00 and is part of a machine quilting set.
The ruler foot is a little taller and thicker than what we might think of as a normal quilting foot. Because the thin ones will ride up on the ruler, bust the needle, and put the timing out. The smaller long arm rulers work best because they are also thicker than our cutting rulers