I got them from SuperRich in Thailand before going. Others that didn’t have USD had to go to the printing shop, who for a fee, would then get a grab taxi to the money exchange and do it for them (the consulate has limited opening hours hence the need for a speedy return).
I believe this info varies from one consulate to another. At HCMC, 6 months statements are asked for. I didn’t ask specifically about how long the 500K needed to be on the account as this didn’t affect me, but they are very responsive and will reply to emails.