I don't know where in Boston you live but if you are not too far from Warwick you can check out the Harbor Freight store. I bought a set for $10.
http://www.harborfreight.com/cpi/ctaf/retail_stores.taf?f=detail&Retail_Store_ID=260
I know Boston really well cause I go there all the time. Unfortunately, there isn't a Pep Boys in Boston. Have you called the Sears in Burlington Mall? Home Depot, AutoZone, and Lowes do not carry those bits. Or look up phone book for small hardware store in Waltham or Newton or perhaps Framingham. I am trying to think of a hardware store around Boston. If I remember a store tonight, I will PM you. Oh, also call the Black and Decker factory outlet store in Franklin (exit 15 on 495N).