How do I find plenty food in order to replenish my health during battles and non-battles? Is there a special merchant that actually sells reasonable cheap prices for food?
Don't buy food. In fact, the only things in the game worth buying are:
1. Crafting diagrams (for runestones/equipment upgrades only; not for weapons/armor) and alchemy formulas.
2. The base alcohols required to brew the white gull potion (and maybe also alcohest early on, though you can generally loot enough alcohest that you don't need to worry about brewing any yourself).
3. Rare crafting materials, if and when you need them.
4. Gwent cards.
5. A potion of clearance.
6. Balisse fruit, if you're having trouble finding it in the wild and haven't brewed the alchemy items that require it yet.
Anyways, you should have started the game with a formula for the 'swallow' potion, which will heal you when you drink it. So the first thing to do is access the alchemy menu and create the potion. Once you do, you can resupply the potion for free by meditating, as long as you have at least one alcohest in your inventory. You can find alcohest by looting the various barrels, shelves, crates, and chests in the game. Or you can brew your own from the alchemy tab, as well.
Eventually you'll come across a formula for white rafford's decoction, which is another healing potion (and often more useful during combat than swallow). Be sure to brew it as soon as you find it.
Using either potion (and also, effective dodging, parrying, and the quen sign), you should never have to rely on food/drink to heal. Though if you're going to use food, wolf livers are probably the best option as they'll regenerate a good amount of vitality and you can get them for free by killing wolves (which the game will supply in great abundance).