I guess an exchange would be the fastest.

but the best would be p2p (see hodlhodl). don't underestimate what liquidity people have to provide.

Some people do p2p trades as a business with virtual assistants.

you could also use bitcoin as collateral via hodlhodl lend or on centralized platforms - nexo is still growing strong. Just be sure to be able to cover your liquidation position if you go that route.