Funny this should come up in a thread, I literally just replaced my Comcast modem the other day. I purchased one of these:
https://www.amazon.com/ARRIS-SURFboard-SB6141-DOCSIS-Cable/dp/B00AJHDZSI/ref=sr_1_2?s=electronics&ie=UTF8&qid=1485798762&sr=1-2&keywords=surfboard+modem
I got a higher end model, but I believe this one will work too. You can shop around to see what you want in terms of speed, and wireless capabilities, but make sure you check whatever model you pick against the list of Comcast approved equipment.
I will tell you from my experience, you are making an excellent decision. We were paying the $10 a month to rent the modem, and then additional for 100 megabits down. I did a speed test before changing anything, and the old modem was giving us about 30-40 megabits down. We dropped the rental modem, replaced it with a Surfboard modem, and upped our speed to 200 megabits down. I installed the new modem, returned the old one, and did a speed test on our network. We are now getting 233ish megabits down.
And as far as service, there are actually some third party vendors that Comcast will service themselves, including some of the Surfboard modems. You'll have to check which ones they still service, but I know there are some.