bookplate stock?

Not familiar with your locale thus have no knowledge of retail outlets near you, but any stationery outlet will have Avery labels. Such stock is made in endless variety of configuration and, packaged as they commonly are in small lots, allows a selected inventory at modet cost. Worth seeking.

Neenah, Mohawk and probably many other paper makers produce acid-free, adhesive-backed paper in letter-size sheets. You just need to cut it down (or have someone do it for you).

I printed bookplates earlier this fall. I took a slightly different approach. Because mine were square, I cut my own paper (I went light: a 24 lb Strathmore ivory). I also didn’t use adhesive backing, because some/most of the bookplates were to go in books that I didn’t want to damage too much. I wanted the possibility of reversal. I use an acid free releasable Tombow gluestick for adhesive.

Anyway, that is my experience. It was a fun project, combining a resingrave block and type.