Comcast comes into the house and plugs into a little box. The box connects to the router. The router connects to the AirPort Time Capsule. If I want to extend the wifi, do I use another Airport or do I use Comcast extenders? Thanks!
The answer will depend on whether your router provides your WiFi network now, or whether the Time Capsule provides your WiFi network. They might both be providing a WiFi signal, but you might only be using one for normal WiFi service.
If your router provides your WiFi network, then you will need a Comcast extender, assuming that Comcast furnished the router.
If the Time Capsule provides your WiFi network, then you will need an Apple AirPort extender.......ideally an AirPort Extreme, but an AirPort Express would work as well (with slower performance).
AirPorts are still available from eBay as new, refurbished or used.