From the What legoes with what thread:
So, the Astrolux S41 tubes should work for you, and those have been shown to be compatible up to S42 and S43 as well. If you get a tube from any of those, it should fit your A6. I haven’t tried it. Although I bought four BLF A6 from the GB, they all went to other people eventually. I have a S42 18350 tube you could get if you wanted to try it. The other concern might be color matching. Since they’re possibly the same maker, that might not be a problem. But I don’t know.