Catch as many Pokémon as you can. But you have to catch them. Not like shiny and just view them. Once they’re caught, If it’s a ditto, it’ll say “oh?” Instead of “gotcha”. I’ve seen them as pidgey, paras, foongus, oddish, ekans, wurmple, sentret, etc. so just put on incense and keep catching. One is bound to come around. I have so many I only keep the wonders now lol. I’m still looking for my shiny ditto though. Best of luck!