Comcast does not sell digital boxes. They only lease them. If you purchase one (say from EBay), Comcast may simply refuse to activate it; meaning you just bought a paper weight.
A Comcast subscription should provide you with two digital boxes. If you require more, you should be able to lease additional boxes at the regular fee.
Now if you really want to go toe-to-toe with Comcast, you should be able to request the specifications Comcast requires in a set top box, purchase one, and request they allow you to use it.
Your best bet would be to contact Comcast and tell them that you want to purchase a set top box and that you need the specifications they require for activation. You may need to jump through a lot of hoops (transfers to higher levels of tech support) to get this information.