You must have access to the second tier of quests in order to do this. Select the Fake In Yellow quest and go through the first three areas with Rappies, killing them off. Once in the fourth area with Rappies (where you meet the doctor in the Rappy costume for the second time), there should be a huge amount of Rappies dropping down. Drop a Telepipe and examine them all to see if any of them is an Al (or a Pal under the ultimate difficulty setting) Rappy. If not, enter the pipe to town then quickly return to the area. The Rappies will drop again. Repeat this procedure until eventually one of the normal Rappies turns out to be a rare Rappy. It is recommended that you have the ability to use Ryuker when doing this, as it is cheaper then going back and forward to pay for Telepipes (350 each).