It's important to clarify: from any authorized store or streaming service. The movie is available officially on 4K Blu-ray, and digitally via platforms like iTunes/Apple TV, Amazon, and Vudu (in 4K HDR), but not as a downloadable IMAX Enhanced file outside of physical discs.
The true Holy Grail for collectors is a —a fan-edited version that stitches the 1.43:1 IMAX ratio (sourced from rare digital intermediates or open matte broadcasts) with the 4K color grading of the retail disc. Interstellar Imax 4k Download