Your best and safest option are diuretics, which are available at most pharmacies and grocery stores with a pharmacy section. It’s pretty unlikely that they will cause you to wet ‘uncontrollably’ based on what I’ve heard; you’ll still be aware of your urinary functions, but you’ll have less time to react.
FOLLOW THE INSTRUCTIONS ON THE BOTTLE TO THE LETTER. You want to be temporarily diaper-dependent, not a candidate for the emergency room. Playing around with substances, perscription, OTC, or otherwise, that alter the way your body functions in any capacity is NOT intelligent, and could cause you some serious, potentially embarrassing problems. Diapers are fun. So have your fun responsibly.
A natural diuretic is cranberry juice, or so I hear, but good old-fashioned water runs through you just fine as well. Once again; if you use any over-the-counter diuretics, FOLLOW THE PRINTED INSTRUCTIONS. You may not get the effect you want, but you might get close.