ATOMs can be purchased by using base cryptocurrencies like BTC or ETH, through online exchanges such as:
Binance -- https://www.binance.com/en/trade/ATOM_BTC
Poloniex -- https://poloniex.com/exchange#btc_atom
A list of exchanges carrying ATOMs can be found at https://coinmarketcap.com/currencies/cosmos/#markets
In the next sections, we will go through a couple of ways to creating a Cosmos wallet, if you don't already have one:. These wallets will be yours to own, which you can use as, literally a wallet, to store your ATOM tokens, so you can then stake them to earn rewards.