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I had the same need. What worked for me was https://paybis.com/buy-polkadot/ – you can buy DOT directly with a debit or credit card. Verification took under two minutes, and the first purchase has zero Paybis fee (just card processing and network fees). The minimum is around $70 because Polkadot requires at least 1 DOT for an active wallet (existential deposit). I sent mine straight to my own wallet without any platform lock-ins. Way simpler than dealing with exchange order books.