I know there are different types of pH calibration solutions, but which one(s) should I be looking for and where should I be buying from?

My Milwaukee pH probe came with calibration solution in a sachet, but with no re-order instructions of any kind.

I know I could always call the good folks at Milwaukee Instruments - Manufacturer of pH, EC, TDS, ORP, Dissolved Oxygen meters, monitors, controllers and testers. but I wonder, what's the /inside/ skinny on this topic? Can these solutions be had for lower prices or of better quality somewhere else?