Charity shop leather is fine for practicing but you'll find it is chrome tanned and therefore won't wet mold into the shape of the knife like veg tanned leather does.
Old handbags and old leather coats make great possibles pouches and fire kit pouches though.
LePrevo leathers in Newcastle really are the best to get your veg tanned leather from. Because of all the people from here buying sheath leather, they are even cutting complete hides into six inch strips to sell by mail order (they just set their strap cutter to maximum width), so you don't even have to buy a whole shoulder any more. Give them a ring and tell them what you are making and they will keep you right on what you need. You'll find them very helpful, and these people don't just sell leather, they make stuff from it as well so they know what they're talking about.
Here's a link to LePrevo's web page on how to make a knife sheath
http://www.leprevo.co.uk/sheaths.htm
Eric