There are 386 different items including Pokemon's specific items, and I only have 277.
I still need 109 and the Pokemon's specific items to complete it and I don't know what items I need.
Is there any place where I can see the items I need?
This isn't what I'm looking for! Stop posting this as answer!
Also if you don't know about PMD, don't answer.
Unfortunately, there is no intuitive way to figure out which items you have and which ones you haven't gotten yet. So unfortunately. You'll have to just go through every single item listed on the Serebii page and attempt to collect it. If the item can't be found, you've probably already gotten it and you can move on to the next one.
I'm sorry that there's no easier way of doing this (besides resetting your game). Good luck!