I had to switch to Mannitol after I ran out of my Caffeine to smoke opioids. I used to buy it from a company doing supplements. All the other ones I tried or bought online, had always whatever excipients or other dietary stuff inside with a shitty taste.
But yeah could be on amazon/ebay there is the pure caffeine not related to supplement for sports and such, make sure it's unrelated to them and is only pure caffeine.