They're with the rest of the terrain tiles, but aren't super obvious
I've labelled the tiles to show how to create the different inner corners here:
184 days ago(+1)
I'm adding blob tilesets soon that will hopefully help solve problems like this. I've tested out the grass so far and it works well. I'm going to work on the other parts after the weekend when I get some free time.
The highlighted tiles are the main inner corner tiles. Anything with that small collection of pixels in the corner of the tile is a corner piece.