glad you scored some. no need for this info now, but perhaps others can gleen from it.
hobby stores commonly carry sheets or rolls of a film used to repair hollow core wings of RC aircraft. gonna see lots of bright colours useless for growing, but they have "silver" as well. its not called mylar, but its the same effect.
ALSO... open a bag of potatoe chips. look inside. hey its mylars cousin. sure, one would have to splice bag after bag together with reflective aluminum tape, but so what. its gonna get thrown in the trash anyway. recycle those munchie scraps. wash the tater grease off first tho...