It it's a single one off channel or two (HBO Go, Apple TV, Netflix etc) you'd likely be best checking their websites. Roku doesn't jack up the price on them, so they're as-is. If its multiple channels like Walsh Felz posted, especially traditional cable channels (ESPN, Fox Sports, AMC etc) you're likely best searching packages which include these bundled. Services like Sling TV (cheapest, $19ish), Hulu TV, or YoutubeTV ($49 I believe however tends to include the most, may require add-ons such as latino or sports upgrades I'm not sure). Also, generally outside of ESPN & Fox the cable channels like TNT, USA, AMC etc do not offer their channel as a one-off.
If you have a specific channel or purpose in mind please post.
