Physical and digital options have been available since March, so you shouldn’t have any trouble finding a copy.
So, where is 1917 streaming these days?
If you are more into physical media and want to buy an old-fashioned disc to enjoy the story of two British soldiers attempting the impossible, then you can do so by purchasing 4K Blu-ray, Blu-ray, or DVD versions of 1917 on sites like Amazon as well as most major retailers.