Man of Honour
Or... Just use coffee which most people have available at home so no need to buy extra stuff....
I'd respectfully disagree and state that coffee is a cheap viable alternative to buying supplementation.
I agree that caffeine supplementation offers more potent effects. But they are not necessary at the same time I'd you're not one to buy supplements regularly.
Either your adenosine receptors are fried from having drunk too much caffeine or you're not having strong enough caffeine.
Personally I'd go cold turkey for a while and let your receptors recover. I thought I didn't respond but I did that and took stock of my caffeine ingestion and it was higher than I thought. Anything over 70mg should have an effect. So having something at over 150 should have you buzzing.
How much tea do you drink? Tea does have caffeine in, even if it's not as concentrated as coffee.