STAKE exists on both the Ethereum Mainnet and the xDai chain (with a different token address on each chain). See below for ways to acquire depending on the chain.
Additional exchanges listing STAKE are shown on the CoinGecko page.
1inch.exchange DEX aggregator https://app.1inch.io/#/1/swap/ETH/STAKE
You can bridge STAKE on xDai to STAKE on Ethereum with the Omnibridge.