Every campsite in Glacier in peak season will get full. Use this website to see what is still open each day and historical info on when the campsites fill up (sometimes as early as 8 a.m.) http://home.nps.gov/applications/glac/cgstatus/cgstatus.cfm

St. Marys is a good bet because of all the privately owned campground in the town. Johnson's is nice and has a great little café. There is also a KOA. St. Marys hedges your bet on options, rather than hoping Many Glacier is open (for example). Also great access to the Going-To-The-Sun Road.