Media Store content would have a Buy and/or Rent button. Some titles may be exclusive to third party (subscription) content providers, and won’t be available for purchase in the Store. Some titles can be pre-ordered, waiting for a future release date.
On some platforms (Fire TV, Android TV, Google TV, Sony TV), the Buy or Rent option may not be available directly from that device; then you may choose to do the transaction from a different device (e.g. iPhone), then later watch on the no-transaction device through the matching account (Apple TV app﹥Library﹥).
When you find a title of interest, try scrolling down on its main details page, until you see a “How to Watch” header. Find one or more available options listed there. The top of that screen might just list one default access option, which may not be what you were seeking.
If we knew the title and your country, then we might check to see if anything special is going on with it.