See https://pathofexile.gamepedia.com/Damage_conversion.

[[The Goddess Scorned]] gets you 100% of the way there by itself, but I'm guessing that's not really what you're looking for. Other than that, you can fairly easily pick up 50% phys to fire from various unique weapons. One of the Watcher's Eye Anger mods can give you an extra 40%. This gets you 90% of the way there.

If you absolutely want everything to be fire, you'll have to do some phys -> cold -> fire conversion. The easiest source of phys -> cold is [[Hrimsorrow]] or [[Hrimburn]] which gives 50%. You can also use a specific Hatred Watcher's Eye for 25 to 40% phys -> cold. You need your total phys -> cold and phys -> fire conversion to sum up to at least 100%. From here, you need to convert cold -> fire. [[Pyre]] gives 40%, so you can get 80% from wearing two. Using the Cold to Fire support gem also gives 50%. If you have all three, all your cold will be converted to fire.

Realistically, I'd say that you're much better off skipping the cold -> fire nonsense and staying at ~90% conversion.