can cats be allergic to only certin type of grain?

hi, my year old female cat's been scratching lately since we changed her food. So i took her to vet, and the vet said that she has food allergy. So we came to the conclusion that she is most likely to be allergic to wheat.

So now, if shes allergic to 'wheat' is that the only type of grain that she could be allergic to, or is it basically including other grains such as corn and rice..?

i really need this information to choose which food i should feed her.. and the prescription diet is too expensive for her 'forever' food, so please help!