1. ZN, the 10 yr notes has 2 to 3x the volume of the 30yr. And the ZF, 5yr notes has almost double the 30yr.
2. ZB (30), ZN(10), ZF (5) are all electronic - US, TY, and FV are the pit versions.
3. IB or any direct access broker will work - I use IB, only electronic markets are traded on...