I've bought pure caffeine in powder form from the Apotheke - although they had to go in the back and rummage around, finally finding a jar that looked to be a few decades old. So, I'm not sure that it's a generally stocked item.
The nice pharmacist did want to know what I planned on doing with the caffeine before she sold it to me, though.
(An article in Nature looked at the use of caffeine as a natural pesticide - so I wanted to try it against the infamous Swiss slugs.
It didn't work.

I think the caffeine just did fantastic things for the slug's concentration, allowing them to destroy my garden more efficiently

)
But you might ask your pharmacist if he/she can get you the powder form.