Depends on what you’re trying to do. The simplest being you can buy an portable A/C, Heater and temperature sensor and hook it up to a SmartPlug which can all then be managed by a compatible Thermostat SmartApp to control the temperature in the room.
Or are you looking for a A/C / Heater which can natively be integrated into SmartThings, your options here will be a lot fewer.
Most A/C units use IR for control. Get yourself a Logitech harmony hub (currently £49.99 on Amazon) which integrates with Smartthings and then use this to trigger heating/cooling activities. That’s what I do with my LG ArtCool split system.
As mentioned by @Shinedown78 most AC systems use IR remotes for control. They can be therefore controlled by Logitech Harmony which has integration with Smartthings although I am a little dubious about how much AC specific control this would expose.
All the various ‘smart’ systems for controlling AC systems that I am aware including Tado Smart AC, Sensibo, AirPatrol and AmbiClimate also use IR to control the AC system. Some of these can be used with Smartthings either via apps or IFTTT.
However another approach is specific for the Daikin range of AC systems. Many of these support a Daikin module for controlling their AC unit via their app over WiFi. There is a matching user contributed Smartthings app.
Since this is a direct link it would in theory be more reliable and allow some bi-directional reporting like its own temperature setting.
Disappointingly Daikin have not progressed their WiFi controller further to have official integration with any smart home platforms other than IFTTT. I would have liked to see them add HomeKit and official Smartthings support amongst others.