Enforcement spot?
 

If a B player takes the ball out of a runners hands 10 yards past the NZ, but a foul was called on B on the other side of the field before the strip, where would you enforce the penalty from. Unlike a fumble where we would have a bean bag to mark the end of the first run, there is no bean bag in this case, is there?

Presuming the foul was a live ball foul during the run, enforcement would be from the end of the run, by A.

That's what I thought, but how do you know where the end of the run is? My question is, do you bean bag when a B player just takes the ball out of a runner's hands?

Sure, you should, and your play is a perfect example of why. By definition, this is a fumble. (2-18)

A bean-bag is NOT a flag, and is a tool to establish a spot YOU may prefer to recall.

It's far less problematic to retrieve a bean-bag you may not have needed after all, than to wish you remembered to use one, that proves necessary.