ladylove
..just pick a facility, call them and ask what they charge for a cash patient, and tell them you need an MRI of the knee.
I suggest you make sure their price includes the Radiologist's interpretation as well.
In South Florida an MRI may cost your insurance up to about $1500., but you can walk in off the street and get the same one for about $380., so be aware that there is probably a significant price difference.
Most MRI facilities also have contingency arrangements for financial hardship, if that is part of your situation.
Please post back and let us know how it goes for you :-).
Best of luck,
Ladylove