If you look up Carol Gray she has some resources such as videos and stories on her website (looks like you have to pay). Social stories are really easy to write once you get the hang of it. When I potty trained DS I took pics of him on the potty and added in a short story about going on the potty. Social stories should always be written from the child's perspective, hence why it's helpful to add in things in their own environment.
I also recommend using sequence strips that you post near the bathroom (ours is above the tp dispenser). That way you can talk about and point to each step in the process along the way. Ours has drawings/boxes for pull down pants, pull down underwear, go potty, toilet paper, pull up underwear, pull up pants, flush, wash hands. Then the hand washing one above the sink takes over. Oh, and, you may want to laminate it
You can google image search "sequence strip bathroom" and they come up.
Re: social stories