If you want buttons and stuff like that you go to encampments. Encampments are harder to find and it is not illegal to relic hunt on them. You just need the owners permission. The trick is to research on letters and sometimes a soldier will say the regiment camped on so an so’s farm for 2 weeks, then you have to check records but you can find it if you try. If might look like a deserted field or even under agriculture today but a long time ago a regiment camped there and they usually dropped a lot of stuff while there. I think a lot of it is from soldiers just tired of lugging stuff around.
Not the oldest thing, but I’m related to Sadie Bushman who was 9 at the time of Gettysburg. The Bushman Farm was on the battlefield and I have a piece of wood with a Minie ball in it with “my bedroom” written on the back.