It used to be the luck of the draw when asking to get 100 new bank notes . Today I specifically asked for them and visit 3 banks next to one another in a mall, all said they only have old notes. So I said okay old is fine as I needed smaller notes, and they didn't have enough old ones so magically they bring me a nice stack of new crisp ones. I'm assuming all of them have new notes in the vault or in the back. Are they reserved for business accounts or special customers? I don't mind paying a fee to get the new ones consistently.

Where might you have found these new bills? USD?

I've found new notes are dispensed from ATM machines. I've occasionally had new notes handed to me when withdrawing from inside banks. The other way to get new notes is from the bank (which I think is part of the Thai treasury) located in our provincial governors building. I've also found when coins are in short supply at banks the bank in the governors building always has plenty.

There are currency dealers for collectors on Charoen Krung Road in Bangkok near the old Main Post Office or at Chatuchak markets if that's your interest. Obviously they would sold at a mark-up to face value.

Yes, new notes, if any, are mostly reserved for customers who ask and reserve them in advance,

mostly Thai Chinese customers,

Other than that you didn't say you're looking for new banknotes.....

But when you use them they go down in value.