Can anyone that has gotten to the Volcanion/Heatran event let me know if you only have one chance to catch them there, or can you find them again later on?

You can most likely catch the other later on as Lunar said that you can get every single pokemon without the need of every going online to trade or use the friend safari.

You can catch both regardless of which one you catch first during that double battle. I won’t spoil the locations for now but they both spawn in different locations later depending on which one you catch first.

How do you get to volcanion and hetran?

Beat the league once and head to Rose Mountain.