Turned out that yes, it was possible to order it from Purism. I did so a few weeks ago and now it arrived!
This one looks almost identical to the one in the phone, but it does say (on the other side of it) “BM818-E1” instead of “BM818-A1” – so far, so good. Now let’s hope the phone doesn’t go up in flames when I try to replace this myself. 