Where can I purchase blank double sided glass epoxy PC boards for making
homebrew enclosures?

I find hamfests to be the best source; Findlay seems especially good.
Unfortunately, you can rarely find any at the smaller 'fests. Whenever
they
are at a hamfest, Far Circuits often has blank scraps at extremely good
prices if you don't need large pieces.
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Indeed Hamfests are a good source ,surely on the European continent where
scraps of suitable sizes (for your purpose) are sold for approx equiv US$ 5
to 8 per kilogramme. Lately ugly construction is on the increase also due
to availability of scrap PCB material (usually double sided).

Frank GM0CSZ / KN6WH