Where can i find my collected community costumes and items?

You can only find your collected community finds in the Goodies Bag in the create mode pop-it. For the costumes, you must go to the tweak menu of the sackbot that has it, and save the costume it is wearing. The community Stickers are accessible at any time in the community tab of the stickers and decorations button of your Pop-it. Everything else is only accessible in Create mode.

Warning, you may need DLC for the item you may not have, not allowind you to use it.