id recommend starting with fake leather if u can find it, craft stores (like Joann's) and then any sort of discount fabric store might have it :) fabric depot, fabric warehouse, etc. depending on how u feel about germs, u could try to find leather/pleather clothes at cheap thrift stores like goodwill and try to cut those up and use em.
as for charms... craft stores again haha. joann's actually has some pretty cool stuff. any craft store should have charms, beads, crosses, etc.
Also i'd recommend flea markets (!!!) and farmers markets. sometimes winter craft faires, if they have those where u live. u also might be able to find social media accounts of leatherworkers in ur area that might know where to get supplies