Due to the focus on miniatures and tactical movement in D&D 4th edition, there are a ton of high quality tilesets and maps available from Wizards of the Coast,
such as this (they have a ton more available as well). If you're more a print-and-play type of person they do have some old tile PDFs available for free download from their miniatures game,
download here. And of course the cheapest solution is probably to just buy a large sheet of paper with 1" squares, laminate it, and draw your terrain on with erasable markers.