I have tried everywhere for this!
Where can I buy the English Region B Bluray for Arise - Pyrophoric Cult?
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If you are just looking for Region B/2 Blu-rays, then they are available here and here, as mentioned by @Memor-X. If you are looking for those with Japanese audio and English subtitles, the only site I found that sells them can be found here.
If you are looking for a dubbed version, as far as I could find on the Internet, no English dubbed version for this exists. A Reddit user claimed to have contacted Funimation and their reply was:
Sadly, we have not made any announcements about an English Dub of either the last two episodes of Alternative Architecture or the Pyrophoric Cult OVA at this time.
This was over 2 years ago and a recent reply was supposedly similar:
Sadly, we have not made any announcements about dubbing this movie at this time. Please continue to visit our site at http://www.Funimation.com/blog/ as we will post all new information there first!
Funimation is the licensor of this series so I suggest you contact them for updates (I can't as it's not available in my region). Amazon support can't help you much about this one as they can't make an English dubbed version available in their stores if it doesn't exist on the first place.