what are some of the cheaper foods you can find in the user shops?

Are you looking to feed your pets, or work on the Gourmand achievements? Using the Resort is the cheapest way to keep your pets fed. As for the achievements, when I started I just went to all the food shops and bought up everything I didn't already have. Now I use Subetalodge's checklist feature to keep track of what I don't already have, and I restock from food shops using that, mark the foods off as I go and sometimes just go through the entire list and whatever is cheap enough I buy from user shops.

You can pick a free fruit from the Ikumoradeekanox tree every day too. The fruits from the tree tend to be cheap in the users shops too.

i am looking for foods to feed my pet to battle them

In that case, it would be cheapest to just use the resort I linked (or the one on the Omen Islands if you can). It keeps your pet full AND happy for less than most single food items would cost you.

Thank You.