Finding other unscrambled channels?

I live in a dorm room in Jacksonville, FL. We are provided with comcast cable. Judging from the channel line up and some of the channels I do get I have the HD starter package. I have a HDTV that picks up some HD channels but I feel like I am missing several channels. I do not get channels like cartoon network, or history which are channels above 100. I do however get HD channels like HGTV, WE, Palladia, FSN and all the local channels but am missing many others that should come in the same package, such as ESPN. I don't understand why I seem to be getting some of the premium HD channels while not getting others. All of the HD channels and some other the others come in on sub channels (ex. 73.1).

I have a Dynex and it will only autotune itself it wont allow me to manually search for channels.

I'm wondering if there is anyway I can get these other channels that I believe should be unscrambled as well. Thanks