OTC caffeine pills: 250-500mg caffeine (about 3-6 cups of coffee equivalent), with the difference being it's much cheaper than coffee, and more deleterious to your stomach. Less liquid, so it's much easier to dehydrate yourself, or get to dangerous (2-3g) caffeine levels. You might actually notice minor-league OD symptoms off 2 pills (500mg), and definitely at 1g.

Really, a bad idea. Modafinil (provigil) or ritalin (methylphenidate) or adderall (d-amphetamine salts) are probably safer physically than caffeine pills, but potentially more addictive.