Where can I get a soil sample for my lawn that is becoming more moss than grass?
A soil test is a good start. Here is our publication on soil testing, including a regional list of labs that can complete the analysis for you: http://extension.umd.edu/sites/default/files/_images/programs/hgic/Publications/HG110%20Selecting%20and%20Using%20a%20Soil%20Testing%20Lab%20w_chart.pdf
Most importantly however, is to understand more about moss. It doesn't come in and kill the grass, it is just able to out-compete the grass due to the conditions on site. If the underlying conditions which favor the growth of moss are not changed, such as too much shade or wetness, soil compaction,poor pH, even if you remove it, it will come back again. Here is our publication on Moss Control in Lawns, which will help you further :