Is there a natural? Alternative? For dexamphetamine?

No. . "natural" stimulants are exceedingly more dangerous to use and monitor than dextroamphetamine. Most contain ephedra that is a dangerous chemical. Ginseng is also a common stimulant purchased over the counter. It is also much more dangerous than dextroamphetamine. Otc drugs are not properly regulated so variations in concentration & effectiveness are widespread. This alone makes them dangerous.