Unit sprites up to v3.10
Unit sprites v3.10 to v3.14
No idea if the 3.15 new sprites are available anywhere around here. If you're new to modding,
The Unofficial Reorganized Modding Manual is your friend, because it is more user friendly than the modding manual provided by Illwinter. The content is the same, it has just been organized categorically instead of alphabetically. Download the attachment from that link.
For new sprites, you either need to modify the existing ones or make new ones from scratch and then place them in the appropriate mod folder for your mod. See the manual on how to do that last.