I bought mine with PayPay through Blockchain Financial. It was fast and easy to do. Fastest and easiest exchange I've use so far. Limited coin offerings though. Should have more coins available in the future as I think the exchange is fairly new.
CoinGecko is a good site to use to find the exchange for any particular coin. They are listed at the bottom of each coin's page. Usually the most active exchange is listed, but there are usually others that you can use.
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