We have had the best luck finding the silver mylar at florist shops, they use
it around the flower pots.  Usually one side is colored and one side silver.
 A buck usually gets me a yard or two ... 
I personally prefer the blue/silver combination as i can use the blue for my
tank back grounds and the silver to line my lighting reflectors.   I usually
just glue the mylar to the reflectors with a craft glue  called "tacky" glue,
it gives me time to work out the wrinkles before it dries.  One experiment
with a spray adhesive proved that i dont want to do that again, there was no
way to work out any wrinkles.
One comment on the ability of silver mylar (yes, even the cheap stuff) to
reflect light:  line a stock light hood from a 10 gallon tank with it and
then compare it to one without they mylar.   I think the improvement is
certainly worth a few cents.
Steve in Kennewick Washington