There is a vendor or two in the beach camp, and the crazy hermit in the cave near the entrance to the island, but other than that...
Unfortunately, there isn't anything you can do. That's one of the oversights in the level-up system - the vendors don't restock stuff on level-up, and even if they did, the fact that you NEED to level-up before they restock in the first place can be frustrating.
Aside from running away and waiting for your health to regenerate in it's own, You'll need to rely on your Healing spell, you should be able to have that now, and maybe points into Potion Efficiency or Master Herbalist. Once you get the tower you can respec those points into something you prefer.
You're not far from acquiring your Battle Tower, but I'm not going to lie to you, the battles on Sentinel Island are some of the most brutal in the game.
You can try walking into Orobas Fjords and levelling up there, you shouldn't be too far below the enemies level there.
