itch.io

The tilesets and 90% of visuals (not including KeyPick assets) were made by me, and you can feel free to use them. I think the quickest way to nab them would be to get a GMS decompiler such as UndertaleModTool, unzip the game exe (it's a self-extracting exe so you can do that), and then open the data.win file with UndertaleModTool to look at all the assets. As for the songs, those are credited in a readme file, and you can probably find them through more conventional means.