Hi, Dan,
I wanted some sheets like you are referring to for a LARGE project and the price was too prohibitive for that project but,
for 6" numbers, maybe a sheet like this would be worth the price.
https://www.lowes.com/pd/SANALITE-POLYETHYLENE-0-25-in-T-x-12-in-W-x-48-in-L-Off-white-Plastic-Sheet/1001463508
As you can see these are 12" wide by 48" tall but, other sizes are available. A bit expensive but, they should last forever.
Menards has similar materials and I would think Home Depot or other box stores would also but, the stores advertise
Polyethylene, HDPE, Polypropylene, Acrylic, polycarbonate, and who knows what all so, you would probably have to actually
look at what they have in store and in what thicknesses to suit your purpose.
For house numbers or something like that, these Polypropylene sheets are extremely cheap and would probably work but,
up close they may not be aesthetically desirable. They are fluted or corrugated so the edge isn't smooth.
https://www.menards.com/main/doors-windows-millwork/acrylic-glass-sheets/amerilux-0-158-x24-x-36-thick-fluted-polypropylene-sheets/blucrp24x36/p-1539107081946-c-7551.htm?tid=401167036542638643&ipos=1
God Bless! Spirithorse