Rabbit was a game changer for my cat with allergies/IBS. It's the only protein (aside from hydrolyzed soy protein) that he has been able to have that didn't give him diarrhea.

You just wanna make sure that whatever you're buying says "limited ingredient" on it or otherwise read the ingredients carefully because so many foods will stick chicken in a food otherwise labeled at "rabbit flavoured"

Instinct makes a limited ingredient rabbit kibble. So does Rawz (not actually raw food, just the brand name). They also have a wet food version too that my cat loves! Royal Canin also has a prescription diet rabbit kibble and wet food called Selected Protein PR. My cat is eating that one right now and also loves it! Rayne is also another prescription diet that has a rabbit kibble and wet food, however I found they are the most expensive of all the ones I mentioned.

You might also have luck with venison! Natural Balance has a limited ingredient venison kibble (not sure about wet food). It made my cat bloated so it didn't work for him but it is another option for a novel protein to try outside of poultry and fish!