I just went through a search for an XS fatbike myself. There aren't a whole lot of them.
I got a Motobecane Boris LTD from BikesDirect -- mine just arrived a week ago but I just now checked and they no longer have the XS/13" available. When I ordered it, it was the only XS fatbike I could find available anywhere.
One that are not currently available but will probably come back next fall (other than the ones prj71 mentioned) are the Specialized Fatboy 24" and Surly Wednesday. Also depending on your height/proportions there are probably some that you'd be able to ride even though they are categorized as a S or 15-16". Specialized Hellga for example. Others that used to available in XS that I think have been permanently discontinued (but may show up used) are Norco Bigfoot 4.3 (24" wheels) and the Charge Cooker 24".
I don't know where you're located but pinkbike currently has a few XS fatbikes listed:
https://www.pinkbike.com/buysell/lis...6,7,8,13,14,15
Ebay has the Mongoose Argus 24".
https://www.ebay.com/itm/Mongoose-Ar...id=13044560272