Reskinning Unity 3D Games??

Hello all. So one of my goals this year as a developer was to get into unity 3d games. Been reading and watching tutorials but before I get into starting from scratch, I think it would be easier to start reskinning just to get a idea of how things work.
So my question to you guys is where do you go to purchase assets and characters to reskin these games?

Anyone got an idea?

ChupaMobile or Google Unity 3D Source Codes, im sure something will come up? Another option post a job on freelancer looking for a specific source code (see how much they are charging)