Serious answer:
Lactulose is used to treat constipation and in high enough doses is known to cause abdominal cramping, flatulence, and diarrhea. I believe it's commercially available without a prescription.
Still, how do you expect diarrhea to add more credibility to your illness? Do you plan on having the diarrhea in front of your doctor?, or just by letting it dehydrate you until you are physically ill.
Also, don't wear your favorite pair of pants that day you try this.