The Comcast forum would be the best place to know what 3rd party cable modems also have routers that would work with Comcast.
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ComcastAll of the ones I found at Linksys (BEF series) is simply a router with switch with no built in modem.
It could very well be extremely difficult if not impossible to find a cable modem with built in router that you can buy.
Also important to note that Comcast in no way will support the modem or router portion. You will be on your own with any issues.