Wait till the supply drops colors are orange, this means that the space biome has mutagel nodes on the asteroids. Alternatively if you can’t seem to get it from space, there is an area in Rockwell’s garden that will give you Mutagen, which you can craft back into mutagel in a chem bench. Mutagel is pretty easy to come by once you get the right biome, just make sure you do all the missions you can. I recommend doing the races as they don’t require preparation for like the combat/ arena ones. Hope this helps.