I believe that the generator coach in each of the three NIR liveries shipped with two B5's instead of a B5 and a B4.
Hands up, I'm open to correction on that, I'm basing that evaluation solely on my own packs, and I apologise in advance if I'm wrong!
If I am correct, anybody who has requested replacement B4 bogies from IRM is likely to have one spare B5 bogie after swapping theirs, so there should be people out there with unwanted B5's once replacements start landing.
I've seen several pictures and videos of generators here on the forum. Maybe dropping PM's to those particular posters, and putting a "Wanted" post in the "For Sale or Wanted" area, might be a good idea.