You basically have 3 options to invest in vegan businesses, based on your budget, and level of risk/reward that you’re after:
Invest in a public vegan company – There are some vegan companies with public stocks. I’ve listed several on this page.
Invest in a public semi-vegan company – Some companies with public stock own several brands, some of which are and aren’t vegan. Not my preferred option, but you can consider it.
Invest in private vegan businesses/startups (for advanced investors) – Investing in private vegan businesses is an option, but these are not typical stocks that you can buy and sell whenever you want. I’ll go over your options to find these in this post. These are the highest risk and reward because you’re getting in the ground floor.