I'm a AAA member so get the free ones there. I cash them at cambios as needed. Usually get a combo us $100, $50 and $20's.

They give you the serial #s on a piece of paper, put them somewhere away from the checks in case the checks are lost or stolen. I'll often leave a copy of the serial #s at home too, so I can call home and have someone there handle getting the replacements for me.