I do not believe there is one that is all inclusive for purchasing all of them. Different site/exchanges have different coins. Kinda a pain setting up the various ones and getting verified.
be careful most of those exchanges are based or funded from Asia and as the news tells you they are getting hit by regulators daily, be cautious and dont let any coins sit online.
you can use binance.com hitbtc.com they all work fine but like i said I do not endorse suggest using anyone these days!!! and I am talking about alts, if you want real coins (ETH, LTC, BCH,) at the best price use gdax or gemini.
I hope that helps and any other questions just let me know
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Particular one you are looking for?
I hope that helps and any other questions just let me know