Home Depot, Lowes, etc.,,, soft ropes,,, SOFT.
I believe that you may be surprised at how much you will want.
Do 10 minutes of homework
--> Lie on the bed yourself and estimate the positions of the wrists and ankles.
--> Take a belt or something and wrap it 3 times around your wrist.
--> Repeat for your ankle
--> Measure the lengths needed
Now get a tape measure and see how long it will be from the wrist/ankle position on the bed to the point where you'll be tying.
Now estimate the length needed for wrapping around that point
Your estimates will probably be too small.
Okay, you got rope ? (<I><B>SOFT</B> ROPE</I> Got it ?)
Now, go to K-mart, Wal-mart, Target, whatever, and buy 4 of their most elegant, soft, thick (whatever, you get the idea) wash cloths or hand towels.
Wrap these around her wrists and ankles before you wrap them with ropes.
This will allow good circulation (you'll want the feet to feel everything) while still allowing you to tie her firmly without pain or cutting into the skin.