Edit: I apologize. Cabbit's sprites do not work well with RPGBoss like I thought they would. I thought that the current default sprites were 24x32. But that is not the case. They are 32x48. Cabbit's sprites appear very small when used with 32x32 tilesets. It would take quite a lot of editing to get them to look nice.
I want to recommend Cabbit's sprites & derivatives for use with RPGBoss. I have actually already begun creating a branch/fork where I plan to replace the default sprites with these. Sprite dimensions are compatible, but frame ordering will need to be changed. The bases are licensed under Creative Commons Zero (CC0). Complete sprites are licensed under OpenGameArt.org Attribution (OGA BY) / Creative Commons Attribution (CC BY) version 3.0.
Here are some links where these sprites can be downloaded: