I don't see them online, but when my cable modem died on a holiday weekend I went to Best Buy and picked up one of the Comcast install it yourself packages, it was like $169. Provided you package is under 343Mbps the modem buddy215 posted should be fine.
Fortunately when recently Comcast bricked that modem with an update I was able to replace with an Internet only option as I dropped TV and phone a long time ago.
No matter what modem you get you will need to call Comcast to get it working. If replacing their equipment turn it in and then watch your bill to make sure they aren't charging you for it. If your bill suddenly changes check to see if they added the modem charge back. Yes, that is experience talking.