What to feed my 3yr old Maltese?

I've noticed that my maltese has been constipated lately and i think she might be getting hemorrhoids. i have read online that it is possible for a dog to get it. I feed her Royal Canin sensitive stomach formula. I've fed her different kinds of Royal Canin dry dog food ever since she was old enough to eat. I've also noticed that she might be allergic to wheat. I want a dog food that has limited ingredients, good fiber, and definitely no wheat or meat by-products. However, i want to be able to get it at the pet store. Anyone know of a good dog food for my maltese?