I have a few thoughts, but bare in mind that you're probably going to have to do some editing of tilesets. Firstly, hot weather plants are mostly up to you. Mostly when you get to close to a volcano, you're not going to find real plants because it's actually to hot. The area around the mountain will probably be what we think of as tropical plants: palm trees and plants with frond-like leaves. For the more mystical ingredient style plants, I recommend simply changing the color of flowers to be unique. Maybe a flower that is red in the center fading to yellow at the edges? Perhaps make the leaves black and red to invoke feelings of burning coals?
Actual plants that grow in volcanic soil include several species of bright, colorful flowers, a few types of tea leaf (Great for potion recipes), certain types of potato, peas, bamboo (tends to grow in hot climates regardless), even Eucalyptus. Of course, I feel that primarily the alchemy ingredients you'd find around a volcano would be more in the realm of minerals and crystals rather than plantlife. Definitely include some rare magic crystals as that's a staple in fantasy crafting.
For more sprites to work with, there are a few pages in the forum that have free resources (with attribution) that you can look over. It may take a minute to get them, of course, since websites don't last forever, so don't be discouraged if several links are broken. My advice moving forward is to save the resources to your computer with some sort of file name indicating the attribution that is required for future reference. If it's downloaded, it doesn't matter if the website goes down.
Hope this was helpful! Good luck on your game!