Nature already mixes it with caffeine in Green Tea. It is effective for helping you meditate if you dont over do it.

https://youtu.be/LqVkiDN79AE

I would be wary of supplements. So much bad conduct in that industry. Even from GNC, they might not contain what is on the label.

Even better than drugs and supplements is diet. The arteries in brain are the first ones affected by cardiovascular disease as they are very narrow so eat heart healthy. Depression and anxiety are linked to chronic inflammation so an anti inflammatory diet is also good. 

That said, I tried some classical nootropics early on and didn't notice any difference even on retreat. 

Another observation is that meditation has had nootropic effects on me. I am some times astonished when I remember a number from the shortest glance even when I am not trying to.

Green tea will be good for that as stated above. Matcha as well.

If you want a supplement, I've used "Tao in a Bottle" from Dragon Herbs, and can attest to its high quality and natural ingredients.

Tolerance to L-Theanine develop really fast and it had the tendency to give me stomach cramp but I was using pretty large dose. The main interest is to improve sleep so perhaps the best application is to get an occasionnal night of good rest.