You have most chance of finding these planets in undiscovered systems, but there are still lots of them in previously discovered systems too! In the old days, before Horizons, it was not possible to even land on planets, never mind set foot on them. Horizons gave us the ability to land on any planets that had a blue semicircle around them in the system map, and Odyssey added the ability to set foot on them to get First Footfalls in many cases. With Odyssey also came planets with light atmospheres, indicated by the light blue shading around those planets - any that also have the blue semi-circle can be landed on and set foot on - and these are where the plants can be found.
There are many systems that appear to be discovered but the system maps are incomplete. Before the discovery scanners were built into every ship, you had to buy them in outfitting, in small, medium and large sizes. If someone honked a system with a small or medium discovery scanner, it usually only showed a few planets nearest to the star and only the largest - Advanced Discovery Scanner - showed the whole system. Then with Horizons came the FSS where the system map was not populated until someone used it to show where the planets are. Even then, often commanders only use the FSS the 'cherry-pick' ELWs, WWs, AWs, HMCs etc so again not every planet will show. As a result, there are still many discovered systems that have planets that have not yet had anyone set foot on them, so it is worth looking out for those - just select the planet in the system map and the top-right icon will show any First Footfall along with the First Discovered and First Mapped info. If there is no First Footfall shown, get down there and try to get it for yourself! It is instantaneous so you can see it straightaway and don't have to wait to hand in explo data.
