Good news: 'I Was Anastasia' isn't impossible to find if you know where to look, and I actually went through this hunt myself a while back. The fastest route I recommend is using a streaming-availability aggregator like JustWatch or Reelgood — plug the title in, set your country, and it will show where you can stream, rent, or buy with English subtitles. Those services are great because they tell you whether the version includes SDH/English subtitles or if the listing is only dubbed.
If a mainstream platform doesn't have it, check specialty services next: Criterion
Channel, MUBI, Kanopy (library-backed), TCM, or smaller distributors often handle older and niche films and usually include subtitle options. For a lot of older titles, physical media can be the best bet — I've found region-free Blu-rays that explicitly list English subtitles in the specs. DVD/Blu-ray details on Amazon or the distributor's site will indicate subtitle languages.
Finally, don't forget digital storefronts like Google Play Movies, Apple/iTunes, Vudu, or Amazon Prime Video rental/purchase pages; they frequently include subtitle tracks even when streaming bundles don't. If all legal routes fail, fan communities and forums sometimes point to legitimate releases or screenings — just be mindful of copyright. Personally, tracking down an honest-subtitled copy felt rewarding and made the whole watch so much richer.