You might want to talk to your vet and make sure there's nothing that would prevent you from a diet food for them. My senior cat gets bloodwork done annually to check for thyroid, kidney and liver problems.
FWIW, we feed a combination of wet and dry food. One of our cats is 12-13 and one is 4. We give them food at specified times of day and separate bowls, though they do sometimes steal from each other. For the younger cat, she gets Science Diet indoor adult and for the older cat he gets Senior Hairball remedy mixed with the dental health one. He has a LOT of hair. We give them a measured amount vs diet food and the vet is happy with their weights. I found that all diet food did (in the past) was make our small cat smaller. The big cat would bully him and eat it anyway.
If you do not want to buy from PetSmart or PetCo (I personally avoid them b/c I don't support stores that sell animals), you can try Amazon or PetValu or any number of other small or online retailers. Many of them take each other's coupons.
And, just like us, cats need to exercise so if they are lazy like my big cat, try new toys or a light pen or something.