There are quite a few rare items to uncover in the recently/soon-to-be released (depending on if you're playing in Nintendo Switch or Nintendo 3DS, respectively) Mighty Gunvolt Burst. Even with the dowsing item featured in the Switch version, you're still bound to miss something here or there.
Fortunately, Super Two U is here with a handy video guide to the game which will show you where everything is, stage by stage:
It should be noted that in order to get everything, you'll need to have the Air Action, Heat Element, and Impact Element custom parts, some of which were detailed previously here.
If you're looking for the items in a certain stage in particular, there are a list of time stamps in the video's description.