I'm having trouble measuring piston to valve clearance (drop valve method)

Well I hope you have installed checking springs on the cylinder you are checking. Checking springs are just light load springs using just enough force to hold up the valves. You already know the degree range to check. Now all you have to do is push down the vavle till it contacts the piston. Stick the feeler gauge in between the rocker arm and tip of valve. Rotate the engine through the range. If the gauge makes contact with the two parts(roller of arm and tip of valve) stop the rotation. Remove the feeler gauge while holding the valve down on the piston. At the same time use a finger to hold up the rocker arm with the same hand. Now continue to rotate the engine until the gap is at the smallest point. Then measure the gap to see where your clearance is at. Note: If you set your feeler gauge a little less then the min. clearance required and it makes contact. You have a clearance problem and need to remove material from the piston.

Summit racing, Jegs, autozone can order them, Murray's usually stocks them(by me anyways). Most machine shops carry them.

Nearly any old lightweight compression spring will work.

I clayed mine last time, but what you are most likely looking for is the clearance between the outer edge of the valve to the valve relief in the piston. Both parallel to the valve stem axis, and radially from the valve stem at the edge of the valve head. I had much more vertical clearance than radial clearance, and was very surprised at that...