I need to know the exact chance and a link to back it up to make sure it's true. Btw Places where you can find trash cans are Lost Hotel, Pokemon Village, etc.
All Week Trubbish, Garbodor
Tuesday Rotom ( All forms says Serebii)
All Week Garbodor
You could also find Items in the range Common Uncommon Rare
There is no percentage known for that so you won't get a correct answer for that, there are common, uncommon and rare listings though. Pokemon are among the rare things to appear in the trash cans.