Do it at any place that lets you borrow the coin. I’m not especially familiar with the offerings in the market, but I know Binance will let you borrow one coin

Do it at any place that lets you borrow the coin. I’m not especially familiar with the offerings in the market, but I know Binance will let you borrow one coin and stake a different coin as collateral. Borrow the coin you want to short, then sell it, and you now have a short position on that coin.