I've been traveling all over the various parts of MM7 and spent many in game days doing so, but the shops always have the same items for sale. Do the shops ever change items or restock? If yes, what is the trigger for this event?

Thanks

They do change their wares. I don't remember exact frequency, but it should be about 1 month of game time.

I tried resting until a month cross over (month 4 -> month 5 day 1), but still no change in stocked goods (in stone city).

OK, it seems like shops restock on Day 8 of the month, at least in stone city.

Save before entering the shop. Buy all the standard (or the special) items, and a message will advise how many days / hours until the items respawn. Then just reload the save and continue on.

I think it's about 2 weeks from the time you first enter the shop, or maybe 16 days. Shops always respawn after two "training sessions" (each of which takes 8 days) in my experience.

In Might and Magic 7,
Where Can I buy spell books for Wizard?

I can find stores for Priests, like body, spirit, or mind, but not fire, air, or earth.

Where can I find them?

Each School; Earth, Fire, Water, Air have separate outlets. Not every City has all the outlets. In MM7, you start out on Emerald Island with Fire and Air Elemental Magic outlets (it is one of the building you are told you have to belong to "Water Guild Membership" or "Fire Guild Membership" in order to shop. You then need to find in a nearby residence the individual who will sell you a membership to that Guild (around 50). Then you can go back and shop.) The Elemental Shop Owner will sell both books and certain skills if you don't already have them: meditation, learning, etc). The building themselves will have a posted sign for Water, Fire, Air, and Earth.
When you go to Harmondale, you have an outlet for each school of magic but the books will be low level. Other Cities will have various outlets with higher level books. Finally, you get to the outlets with the highest level books when you choose either the dark or light path. later in the game.
Books will also occasionally be on the shelves of magic shops; in treasure chests, or as remnants of monsters you have killed.