How to get a longer coax cable?

Is there anyway to get a longer coax cable for my Comcast Internet service? The place I want to put my modem is too far from the nearest outlet. The coax cable that came with the self-install kit is too short. I went to xfinity store and they don't carry longer cables. They told me to contact customer service which I did. The customer agent promised to call me back within 48 hours to help me get a longer cable but I didn't receive a call back after a week. I talked to another agent today and this time I was told you guys won't be able to get me a longer cable. I was hoping comcast would take the extra step to get customers connected. I didn't know getting a coax cable is that hard for comcast, the nation's largest cable provider. Any help would be appreciated.