Do a google search for Sci-Fit Caffeine, I think you can get a bottle of 200 caps @ 200mg for around $7. WAYYYY cheaper than getting 20 Vivarin for $5 at a store or using Guarana.
Nothing wrong with diet sodas..........except maybe on CKD........too many people are fretting about the asparmate and citric acid, but I'm not familiar with it enough to know if it'll kick you out of Ke.....