Cable boxes are not sold at retail in the US except for Tivo.
You should have come here first and there are many warnings not to purchase a cable box. It's not Comcast's fault you didn't do your homework.
A DCT700 is a very old box for SD service and would not be recommended for anyone these days. Boxes like this are sold by shady resellers with questionable sources on where they were acquired, since there is no retail versions.