This doesn't just have to be in After Effects, as I am keen to learn lots of other software, but After Effects is what I know best.
When creating fire I strongly advice you use a simulation software (blender, Houdini) are what I use
As far as stock footage goes, I use, love, and highly recommend ActionVFX https://www.actionvfx.com/
I second that
For realistic fire you'd have to do your own simulations using either Houdini or Blender. It can get pretty involved and I am yet to create some tutorials for it, but it can yield amazing results.
Second option would be to use a particle plugin like Trapcode Particular. However, the fire is likely going to look a little flat and not as 'real'. Think of video game fires - as it'd be sprite based.
Last option, great stock footage can take you a long way and ActionVFX is definitely the place I'd go!
I also have an affiliate code with them that will get you 10% off if you do decide to go that route - surfacedstudio
But there are lots of options and you may have to weigh up how much effort you want to put in and what quality and level of realism you are happy with