Cablecard Question/Comcast

Hi, I'm new to the forum so please excuse me if this is posted somewhere else. Here's my situation. Out of 24 month directv contract. Thinking of switching to Comcast and getting a Tivo Roamio Pro with the Mini's. (Had Tivo 10+ years ago and have missed it ever since.) I talked with the rep at Comcast and they told me I can purchase my own CableCards (I would need one for each unit?) and then they would give me a $2.50 credit per cablecard on my bill instead of charging me the monthly fee to rent it. How do I go about purchasing a CableCard? and having it be compatible with the unit and Comcast? They couldn't quite tell me that, except that they see a bunch for sale on ebay. Please help? I don't want to pay $21.40 in equipment fees each month for 4 cable cards! Any advice will be appreciated. I haven't bought anything yet - still trying to work out the logistics.

Thanks!