As to this seller on AliExpress, I had a bad experience with them. Bought 2, but both batteries won’t charge. Stuck at 2.5 V. Now have to send them back (that is, if I can find a printer to print the return label 
).
Re: floss-shop.de, isn’t this exactly what we’re looking for? Ersatz Akku Librem 5 Smartphone | FLOSS Shop DE Because they give the impression they are in stock, I’ve ordered 2, I will report back once I receive them. (Isn’t that site run by Nicole Faeber, who was at one point Purism CTO?)