Hey there! What do you mean by "effect item"? Since red/blue doesn't have held items, I suppose you mean items used in battle? In certain battles like the S.S. Anne Rival, they may use an item in battle ~12.5% of the time when their pokemon is under ~20% hp. If this is what you're looking for this spreadsheet has each trainer, their moveset, and the items they may use under Special AI: https://www.dropbox.com/s/hvl96aohy7wq4sz/Pokemon%20Red%20trainer%20guide.xlsx?dl=0
Edit: It was pointed out to me that rival effect is just another name for the Jack item. You can read about it here https://glitchcity.wiki/Rival%27s_effect or ask in the psr discord under #red-blue-glitched https://www.speedrun.com/pkmnredblue/thread/se2sp
Sorry I missed your response, SRC Forums aren't used too often anymore. I am not sure what the easiest way to get the Jack item is since I mainly focus on glitchless categories. Either asking in the psr discord under #red-blue-glitched https://www.speedrun.com/pkmnredblue/thread/se2sp and/or in the Glitch City Research Institute Discord https://discord.gg/EA7jxJ6 would likely get you the answer you're looking for.
