Mercer Island is dispatched by Norcom. They can be heard on the Seattle / King County (KCERS) trunking system. They are best heard on the EPSCA Simulcast site. The dispatching is on the same talk group as Kirkland (19472 - NC POL 2). Listen for the "Ida" or "i" units. Make sure you update your Sentinel database so that it you have the latest and greatest information. You should then be good to go.
If you at all interested in the FD, they too are dispatched by Norcom - listen for the 90 series units.