This is a post for those brand new to cryptocurrency because it's something I see quite often.
Being part of some Facebook groups, I often see this question a lot. "Where can I buy "X" coin?" I wondered the same thing when I first started. But, it's actually quite easy to find out which exchange(s) a particular coin is selling.
First, you'll want to go to Coinmarketcap.com which lists just about every coin that is on an exchange. You can sort the different coins by clicking the different bold headers (Name, Market Cap, Price...).
Once you select a coin, you can view different information about it.
In this case, I'd like to know which exchanges currently list Ripple (XRP) because I want to buy some. Click on the tab which says "Markets". You will see the different exchanges listed under the "Source" header. Pick an exchange from the list, and click on the link under the "Pair" column. This takes you right to the coin on that particular exchange.
I know this was pretty basic, but like I said, I see this question quite often on different groups. So, I hope this helps out those that have had this question.