I'm in the UK so might be a little different from you. I buy with fiat from Coinbase or Bittylicious and then buy other coins on Bittrex, Bitfinex, Binance and Kucoin - depending on what I need. I also do Bitcoin derivative (CFDs) trading on FXOpen, eToro and Plus500 occasionally. Let me know if you want referral links for any of them... I think most of them benefit both the referrer and referee!