Unfortunately, it is not as easy as buying your own intenet-only modem is.
Comcast is very particular about eMTA (voice and internet) modems; you can not just purchase an eMTA anywhere and have it provisioned for Comcast voice and internet.
To my knowledge, the only owner-bought option for an eMTA modem, would be one purchased through a Best Buy that has a Comcast center. Here is a link to Arris about this: http://www.arrisi.com/support/index.asp even then, I would call tech support and make sure that this can be done in your area.
The only eMTA currently available for sale would be the Arris TM722G (which would work fine for you, it is just a more advanced model docsis 3.0 versus the TM502G docsis 2.0)
At any rate, definitely DO NOT buy an eMTA from some other source and expect Comcast to set it up, it will not happen. There has been a lot of angst over this. I am not arguing whether Comcast is right or wrong in this, just stating what the current situation is.
Yep, I understand that. Some folks have had success going through Best Buy, and some have not. Further, just as in the real-estate game, it is based on location-location-location as to whether Comcast will provision a purchased eMTA. That is why I recommended contacting Tech support before buying anything.
At any rate, until there are some changes, that is the only approved way to purchase your own eMTA and get it provisioned by Comcast.
I have a Comcast-supplied TM7229G and would prefer to buy my own and not pay rent on the modem, however, to me, it is more trouble than it is worth right now. I can/will wait and see if any changes on buying and provisioning an eMTA in the future.