There are several missable pins throughout the game so I would certainly suggest following a guide. For the most part, I followed this guide here:http://uk.ign.com/wikis/epic-mickey-2/Walkthrough
Unfortunately, there is no complete guide that seems to cover everything so have your whits about you!
Note that you CANNOT REVISIT Dark Beauty Castle and Yen Sid's Lab.
There is 1 pin in Yen Sid's Lab for "PAINTING the picture at the end of the level." You can also thin it but this will only give you money.
There are 2 pins in the Dark Beauty Castle level. When you get into the tower where you need to work together to power the machines, you want to get to the 3rd machine and paint the wires above it and power it up. This will allow the middle part of the tower to become a series of steps.
Next, thin the same wires on the 3rd machine and the machine will blow up causing a hole in the wall. This displays a pin just behind it. DO NOT go all the way outside, just collect the pin and come back in.
Then you want to climb up the series of steps in the middle to the awning/balcony of the tower above the courtyard. There is a chest on this balcony with the other pin.
(Also note, the other missable collectable in Dark Beauty Castle are a steam boat willy costume. This is found where you throw Oswald up on to a ledge and they break forming stairs. Don't go up the stairs but instead turn around. On top of the chandelier is the collectible. Oswold should prompt to throw you up there)
Farming Scrap Metal:
Hopefully you will have reached the end of the game making the correct decisions in the side quests etc. If you keep following the IGN guide I linked above, you shouldn't have too much trouble with them and making the right decision.
You will need a large amount of scrap metal in order to buy the rest of the pins from the pin store in Main Street North. The best way to farm scrap metal is start outside the clothing store in Main Street South. Across from there, on top of the building, is a large gorilla. Thin the blocks on its arms from a platform below. This will cause a small platform to drop revealing 15 scrap metal.
The routine: Start inside the shop, Hold A and go outside again, run directly across, hop on the ledge showing and it will grab the 15 metal. Hop straight back and back into the clothing shop. And then repeat this step.
100 scrap metal = 5 gold pins.
Sorry if the layout isn't great but this game really is a messy game and I could go on for hours talking about it and advising you on which decisions to make.
If you have any questions or need help, please message me.