You can get 120 pills of 100mg of caffeine for $6 including shipping "Jet-Alert 100 MG Each Caffeine Tab 120 Count". There are fancier ones that include L-Theanine (found in green tea) to mellow the caffeine jitters out but those are significantly more expensive.
Can confirm. Bought two in 2013, still have ~450 grams in the first bag I opened. Well worth the ~$50 (including a jeweler's scale) for a lifetime supply of preworkout supplements.
The FDA banned bulk caffeine sales in 2015[1] or 2018[2] too. This is probably a good idea, most folks have no idea how much caffeine is in a cup of coffee, much less how to measure that dose in powdered form.