I'm not sure where you live, but many larger cities have Asian food markets. You can normally find white, red or blended miso in the refrigerated food section of an Asian grocer. I normally get my shiromiso (white miso) from Ranch 99 (a local Asian grocery chain in the Bay Area). You may also have luck at some of the nicer grocery store chains in your area. If you're on the east coast, your neighborhood Publix will probably have miso paste. Or you could order online, if you don't mind a little wait. http://www.asianfoodgrocer.com/
You can find it in your local Asian market around your town. White, red or blended miso comes in the refrigerated section of your grocer. Try the local big chain grocery stores also, they have all kinds of Asian foods out now.