I worked in a public library for years and received many book publishers' catalogues - often they had really good quality paper or cardstock covers. The back cover was often just a plain coloured sheet so once I was done with them, I'd rip the covers off and save them for layering. I've been retired for two years and I still have a file of these to use up. I also save cardstock advertising leaflets that come in the mail.
Great ideas everyone!
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should you care to look! Or please visit my blog, Cardmaker's Garret.