No exchange platform allows you to buy DOT directly in Dollars ($) as of December 1, 2021
Step 1: It is therefore necessary to go through an exchange platform accepting the Dollars ($),
to buy Ethereum (ETH) or Bitcoin (BTC) from Dollars ($), then you have to exchange those BTC or those ETH for Polkadot (DOT) on an exchange offering the pairs
BTC / DOT and ETH / DOT (See step 2 below)
To buy ETH or BTC from dollars, you can use the following platforms:
Recommended, the exchange leader, very reliable, easy to use moderate fees: less than 3% fees on transactions, for purchase in SEPA and credit card respectively. (Binance on Wikipedia)
Use the link below to sign up for Binance and you will get a 20% fee discount for every trade.
The world leader for casual customer, moderate fees: 3.99% fees on transactions, for purchase in SEPA and credit card respectively. Use the link below to sign up for Coinbase and you will receive a $10 free amount of BTC after purchasing $100 of cryptos.
The credit card is accepted on those 2 platforms, but it is preferable to go through a bank transfer because transaction costs are generally lower than a purchase via credit card
To exchange those Ethereum (ETH) or Bitcoin (BTC) purchased in step 1 for Polkadot (DOT), you can use the exchange platforms of
cryptocurrencies to cryptocurrencies offering BTC / DOT and ETH / DOT pairs
For Uniswap and Balancer , it is imperative to buy and send Ethereum (ETH), because Bitcoin (BTC) is not supported by those platforms
For PancakeSwap, you need to buy Binance coin (BNB), and send it to metamask. Follow the tutorial here : Binance Smart Chain