Here's a secret source for obtaining metal of all kinds. The thing is it is sweat equity. If you live in a moderately big city, or regularly visit one, you can leverage machine shops.
This option is not without risk, and not everyone is well disposed toward hobby machinists. But in almost every case I've been received well. I talk to the foreman about something specific I'm looking for, out of their cutoffs bin. Usually they don't have it. Usually they ask what I make, and we chat. I then ask permission to come back if I need something else.
tips:
Come around lunch or morning break. That's when the supervisor has a better chance of giving you 3 minutes.
Don'tstart with anything exotic. 2" 6160 about a foot long is an example starting point. If they don;'t have any, they sometimes tell you what they typically have, and once in a while you get invited to rummage.
Be very respectful and deferential.
Take no for an answer. you aren't selling anything, you are making a human connection.
As soon as the encounter is over write down who you talked to (including the names of anyone you met), a summary of the discussion, and the outcome. Keep this log for every contact. Review it before the next contact.
I have 4 machine shops where I'm welcome, and one is even collecting extra cutoffs for CHMW members. Another shop has given me open license to jump in to their rollover any time to grab a piece or 2. That 2,000 lb haul of tubing and rod stock including stainless and 4140 rod? Another contact.
All this takes time and patience, but is very rewarding. One time I needed a piece of tubing for a construction. It had to be at least 1X2" 1/8 wall, 24 inches long. A shop had a 2X2X3/16 wall 5 feet long. I told them I could use it nicely. they gave it to me, and told me 'no charge' .
It isn't all roses. Two shops, out of the dozens I talked to, were hostile. I just never went back.