This link contains eBay search results for the Destiny Deoxys DVD.
As I write this answer, the cheapest disc available is $7.91 USD. I think you should buy this movie as a DVD because it ensures you own it permanently (sans disc rot), unlike streaming services or digital purchases. If you are less concerned with the loss of digital media than I am, you can also find Destiny Deoxys on YouTube, Google Play, Amazon Prime Video, and Apple TV. Currently the cheapest option is to rent it on YouTube for $3.99 USD.
