Where can I find (or how can I calculate) the glide ratio for a PA-28-181?
If you were to take off from an airport at 2,000' ASL on a standard day, and climb to 7,280' ASL you would be exactly 5,280' above the ground which means exactly 1 statute mile above the ground. 

If you were to cut the engine, and then use the information available in the chart, you should be able to glide about 10 statute miles which yields about a 10 to 1 glide ratio.