Cats have very, very specific dietary needs so their diet needs to be balanced with muscle meat, bone & organs, each in specific ratios that reflect a prey model:
80%muscle, 10%bone 10%organ.
Feeding too much tuna is not good for your cat as it is not a balanced meal. However to entice your cat to eat, a bit of tuna added to a meal shouldn't hurt.
You won't find those foods at your vet (and don't allow your vet to talk you into their food, it is overpriced, poor quality junk, in my opinion). A good pet food store (Global, here in Ontario) are the places that sell these. Here are their store locations in Quebec:
http://www.globalpetfoods.com/store-locations