Oh no, I am so sorry that this has happened to you and your horse! Post MISSING ads on every semi local facebook horse group you can find. Contact all of the local rescues and explain your situation, even bring the photo to them. If she has a microchip or tattoo #, definitely provide that info to them as well.
I hate to have to say it, but I would also find it disconcerting that the mother wouldn't simply give you the name of the rescue. I would also be attending ANY and all auctions (or begging a friend to go if you can't) to see if you can find her.
Good luck in your search! I would be a complete mess if something like this happened to me. Nikkibella is right, her markings and blue eye give you a heck of a lot more of a chance to find her than if you had a plain bay or chestnut.