Young Josh, Pewter, bone, wood, shell and even lead buttons were not uncommon in the era of the civil war and could have easily found their way into a "Housewife" sent from home. And those of us who have done route marches, slept out campaign style etc know how necessary replacement buttons can be.
My own trouser buttons are paper backed tin and w/ one exception always have been. The one pewter button in my kit is on a shirt and that button is quite neat to me as it is a Civil War button I bought in Charleston, SC back in 1996. It is in fact one of the original items I own. Along w/ my original musket tool it is the only original item I carry onto the field.
After Brian mentioned using a sling as a waist belt I did a little searching and asking questions of collectors that I know. I was suprised to discover how a length of rope was probably the most consistant belt. I was suprised to hear of a set of braces made from a
CS musket sling. Een more suprisong considering the documentation put the trousers as those of a Union man. An interesting use for a bit of battlefield scrounging.