This Seeking Tech guide will show you how to find the Heal and Heal II skill stones in Jeanne d’Arc.
You won’t be able to buy the Heal skill stone directly in shops until Stage 17, starting in Reims.
Until then, you have to get them to drop from enemies like the Mage and Necromancer. You can find these in the Free Combat version of Neufchateau, which opens up after Stage 4.
To upgrade to Heal II as early as possible, you have to wait until after Stage 10, when you unlock the skill binding ability. Afterwards, you can bind Heal with HP +20.
You can buy HP +20 after Stage 6 in shops like Chinon. While you can also use Magic Attack +3, you won’t be able to purchase it until much later in the game at a more expensive price.