The 356 is a common alloy for reproduction Harley Davidson cases wheras the 319 alloy is what the HD factory uses. They both have silicon in them and therefore both really flow good. I think the 319 has some copper in it as well.
Pistons are good scrap because they are a heat treatable alloy and contain silicon.
Usually they are die cast or forged. If your using lawnboy pistons, don't use any other lawnboy parts because alot of lawnboy stuff is magnesium such as the mower body and some engine cases. Most auto shops will give you old pistons. I'd stick to a virgin alloy though. You can get 319 or 356 at Atlas metals in colorado.