Since the best or favorites might not be available, looking at something that would be safe to buy 8 to 16lbs of.

From high level searching I see some good choices but they are also the most popular and thus almost unobtanium at less than $50/lb. Is there anything else that stands a good chance of working well?

I’m not set on the 6BR at this point but most calibers close use the same powders as optimal.

Should I just wait until one of these shows up? It will be a while before I’ve got all the pieces together.

Varget
IMR 8028 XBR
VV N140
H4895

https://www.6mmbr.com/6mmbr.html

I think that linked site has more info than any of us could type here. I’ve not loaded for the round, seems to favor medium speed rifle powders.

To the other question, one would have to start with what one normally shoots & round counts. I’m much more into the medium to faster rifle powder areas, 223, 308, straight wall rifle, 243, & similar. I can always find a use for a 4895,;TAC, 8208, & Varget.

Then we get to what’s available, at a somewhat reasonable price. In the last 18 months or so I bought, 8208, H-322, AA1680, IMR-4198, & HP-38, & some IMR-4320. Just about all was at regular shops, reasonable prices. I see powder at gun shows, $50 or more a pound.

With the right powder, no problem buying an 8 lb jug.

IMR 4166 is slightly less temp stable than Varget but aside from that it's almost identical in terms of burn rate and charge weight. Might open up an avenue or 2. It's my preferred powder for 308.

VV N140 is also pretty close to Varget as well.

No idea if it'll work well for the cartridge, but I've had excellent luck with IMR 4451 in 6.5 Creedmoor.

Shot all four powders in a 6BR as well as a 223. Shoots great in both calibers. If I were in your shoes, would wait for one of the four.

Awhile back bought some V140 here, https://www.whiddengunworks.com/powder/ sign up for email notification.

6bra powders
Varget, R15, N203B, H4895Â,
N540
CFE223 32gr start.
N-150. Imr 8208
reloader ARcomp.
30.6-31.4 gr of N140 

My list so far.
I got 8lbs cfe223 going this year.
103 Roy Hunter & barts hammers

When I had a rifle in 6 Norma BR I loaded 105gr Scenars with N140. My best group @ 268 yards was just under 1/4"