How flexible does it need to be?  I have some PCB material from
"Rogers corporation, Microwave materials division" (100 S Roosevelt,
Chandler AZ, 85224)  Labled "Rogers RT/Duroid", this is perhaps 1/32
inch thick and about as flexible as poster-board (might be a bit more
flexible once most of the copper is off.)  I assume this stuff has a
magical dielectric (all teflon?) and costs a small fortune bought new.
(mine was surplus and cheap, heh heh.)

BillW