Currently, the only way to get the Call of Duty: Modern Warfare remaster, Modern Warfare Remastered, is to pick up premium versions of Infinite Warfare. That might change soon, however.
Since-removed product pages on GameFly spotted by CharlieIntel suggest that Activision might be planning to offer Modern Warfare Remastered on its own. A GameFly listing for the PlayStation 4 version of Modern Warfare Remastered has a June 20 release date, possibly suggesting PlayStation 4 is getting the release 30 days early.
The original listing has since been removed. Activision has not confirmed this nor have they stated yet if or when Modern Warfare Remastered will be available separately.