See if there is a PCB manufacturer within resonable driving distance and
ask for scraps. That's how I got a huge stash decades ago. Single and
double sided, the good stuff (FR4). But it has to be picked up, in my case
they wouldn't have gone through the trouble of packaging and mailing it.
Basically it had to be no more effort on their part than a walk to the
dumpster would have been.

Mine weren't tinned though. Also, I am not sure if constantly touching a
pre-tinned board during experiments would be too healthy.
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Printed circuit board scraps up to large pieces ,be it usually double sided
, are always available at ham radio flea markets in the various european
countries. I think I have stocked up for life ! From the larger bits you
can make neat enclosures.

Frank GM0CSZ / KN6WH