Making aspirin without acetic anhydride I want to make aspirin. Normal method is to react salicylic acid with acetic anhydride, but it is extremely hard to find acetic anhydride. How may I synthesize aspirin with acetic acid and salicylic acid only?

I have watched a video where someone makes it with sulphuric acid, salicylic acid and acetic acid. Can I use phosphoric acid insted of sulphuric acid as a catalyst?
I just looked in the literature, and it has been done even without catalyst in a "reactive distillation column" at high temperatures (patent US201113194873). If you can remove the water formed, you can technically drive the reaction forward, even if it is slow. A catalyst will just make that process easier, requiring less heat and time.

Phosphoric acid is a much weaker acid than sulfuric acid, and it does not have the same dehydrating properties. However, if you have a proper setup, it should do the trick. A big problem with phosphoric acid is that the most concentrated you can normally get is 85%, which means you still have to remove 15% of water somehow. If you don't, you'll likely hydrolyse any aspirin formed and get your starting material back.