I just got some plastic mirrored pieces at Michaels. Don' t know if the spray you're talking about is Krylon Looking Glass, but I just got 2 cans of that at Walmart.
I've used the Krylon Looking Glass as well. It's easy and reasonable in cost. There are other methods of 'mirroring', it just depends on what type of piece you are working on and look you're going for.
I know that this type of product has been used a lot in the construction of DIY solar ovens...because it is light weight and flexible.
the price is not too bad either
They sell the mirror spray paint (Krylon- Looking Glass) at Hobby Lobby & at Walmart.
And they sell it on ebay, so be sure to price ck! I'm all about saving $$ ;) xo