Hmm, I don't know about platinum but in Diamond and Pearl if you use the poke radar in the patch of grass by Route 218. But it can be quite pesky to obtain!
If you don't have any luck finding ditto there then I recommend trying the GTS.
Hope this helps ^^
In the Trophy Garden.
you need to put dito and manaphy in the daycare and they will have an egg
well i only know where you can get ditto in diamond but since there both pretty much the same, id say right outside of canalave city use pokeradar in grass
shiny ditto and get from gba games get nat dex and migrate
You can find it at random days in Pokemon Mansion in route ---, talk to Butler (or whatsoever) and he will say random Pokemon is in the garden, if is ditto, go get it!
Go south from Goldenrod and once you get to the the daycare walk in the grass around it. I found my ditto there.
trophy garden
hey you cant get the move transform unless its on a dito,mew or on mewto otherwise if u have action replay POKESAV IT
Giant chasm (outside). Go into thick grass because you can get lot strongest dittos there from level 63- 65.
ditto...go to canalave then go to the patch of grass outside it.use ur pokerader. (it may take a while)
you have to first get the national dex and then go to the Pokemon mansion and talk to mr.backlot he will say that he saw a cute ditto. it doesnt work all the time. you should try everyday. there are new Pokemon each day.
You have to catch a Dito with the same nature as the legendary and pute them in the daycare. You catch Dito in trophy garden.