Try your local flea market or second hand store. My wife and I bought 5-6 converter boxes last year here in Dallas at a flea market for $10 a piece. Gave a couple to our family members and kept the rest for ourselves. They're all still working.
Depending on your budget and patience level for walking around flee markets, you can get digital converter boxes at Amazon for about $30 USD or thereabouts. If you want to up your level of viewing enjoyment by a level or two, you can find lower-end digital converter boxes that also have PVR capability for about $60-$80 USD.