Hello Jeff and welcome to the site. First and foremost welcome to the world of segmenting. Not sure if you are talking about using materials in knife making or pens or maybe both. But it does not matter too much in that most materials are interchangeable and can be used to make most anything. I will refer you to the the forum "Blank Making" and click on the sub forum "segmenting". I do this because there is a world of information and surly answers to your questions within there that we will not touch on all of them because it is impossible. You will find this helpful I guarantee it.
With that said I will get the ball rolling and others can add to this thread as they see fit. For me there are so many materials that can be used for segmenting and just casting in general. But being you were specific with segmenting lets stay there.
Metals of all kinds-- aluminum, brass, stainless steel, bronze, and a few others.
Plastics--- credit cards, guitar picks and guitar guard or pickguard material as it is called, acrylic plastic sheets, can lids, wrapping paper, corian
Paper--- stock of various thickness, box material, cardboard
Materials--- blue jeans, scarfs and many others
Wood--- veneers of all thickness and dyed or colored, woods of various species cut and planed to thickness
Here are some places I get all these materials
Home Depot, Lowes
Lumber yards
web sites so many
https://www.speedymetals.com/s-137-sheet.aspx
file:///C:/Documents%20and%20Settings/Administrator/Desktop/New%20Folder/Pool%20Cue%20Veneer,%20Dyed%20Veneer,%20Colored%20Veneer,%20Marquetry%20Veneer.html
file:///C:/Documents%20and%20Settings/Administrator/Desktop/New%20Folder/Hardwood%20Dowels%20-%20Woodworkers%20Source.html
https://www.cwp-usa.com/
https://www.delviesplastics.com/c/acrylic-plexiglass-sheet.html
file:///C:/Documents%20and%20Settings/Administrator/Desktop/New%20Folder/Dyed%20Veneers%20-%20Thick.html
file:///C:/Documents%20and%20Settings/Administrator/Desktop/New%20Folder/PLAIN%20OPAQUE%20BLACK%20STYRENE%20SHEETS%20-%20Evergreen%20Scale%20Models.html
file:///C:/Documents%20and%20Settings/Administrator/Desktop/New%20Folder/Schmelke%20Cues.html
I have so many more but will others add theirs if they choose. To sum this up google is your friend when it comes to finding things. Use your imagination for materials. Use the segmenting forum for inspiration. Good luck.