Usually you don't have just a "Decoder", rather you have an all in one box that tunes the signal, de-muxes the packet streams, performs conditional access (for encrypted channels/providers) , operates a UI and system housekeeping, and "decodes" the tuned streams into video and audio.
That said, if you have a pay TV provider, you need to get the box from them or who they say to get one from, so they can authorize it for the programming you are subscribing to, and it be compatible with the signal modulation and video formats they use.
For broadcast TV, you need either a TV or set-top box that has a digital tuner/decoder which supports the format your area uses. Such boxes can likely be purchased from local retailers.
For Free-To-Air satellite (which must not be confused with receiving encrypted services by defeating conditional access encryption, which is pay TV piracy), you need an FTA receiver and dish compatible with the satellites channels you want to receive.