standard definition tv shows on latest Apple TV (HD) ?

You can get the SD version of content, but such purchases are not eligible for HD playback at any time in the future (it is a different item to buy).

Settings﹥Apps﹥iTunes Movies and TV Shows﹥Limit Purchases and Rentals to SD

Alternatively (1): All purchases are linked to your Apple ID. If you find it easier to make SD purchases in iTunes on a computer, or on an iOS device, then you can do so. Those purchases would then be available on all your devices with that Apple ID, including the Apple TV, from the Library or Purchased tab.

Alternatively (2): Settings﹥Apps﹥iTunes Movies and TV Shows﹥Video Resolution﹥Good (SD)|Better (HD)|Best Available (adaptive up to best possible). This Good setting without the Limit to SD, could facilitate low bandwidth without giving up future HD rights.