You can use any charger generic, PC, Mac - 5W original iPhone, 10/12W iPad charger, 18/20W newer charger, or what I have lots laying around: 29W (Macbook 12" USB-C), 61W (13" Macbook Pro T2), 81W (15" Macbook Pro). 100W (16" Macbook Pro).....
20w minimum to get the full 15w charging speed. If you use the 18 I think you’ll still be “quick charging,” but will only reach 12 or 13w if I remember correctly...