Why exactly you want buy flash BTC? So far there is two definitions of flash BTC. Which one you need exactly? The first definition seems using by scammers,
The traditional definition of flash coins are cryptocurrencies that are briefly sent to your wallet but don’t remain there for long. They can be any cryptocurrency, including Bitcoin (BTC), Ethereum (ETH), Ripple (XRP), or Solana (SOL).
Scammers or fraudsters often send flash coins using Peer-to-Peer (P2P) trading methods, where cryptocurrency is bought and sold directly between users.
Do you want something like this? Then why looking for someone else? You may do by yourself. No need to ask anyone for sending flash BTC or USDT. But it seems this isn't have using for good intentions.
Here is the another definition of flash BTC,
Flash BTC is a technology that enables users to obtain Bitcoin (BTC) instantly on the Ethereum blockchain through a trustless mechanism, without having to wait for confirmations or pay high fees associated with Bitcoin transactions.
Synthetic BTC tokens are created on DEXs using Flash BTC as collateral.
This sounds like WBNB use on the Dex but I am not aware about USDT. I know there is WBTC as well that's not real BTC. They are on some other chain just like a useless tokens.
Just curious why are you looking for this kind of service? Also clear which flash BTC exactly are you looking for ?