For cheap mylar (kind of thin and a bit delicate but it works just as well) go to an outdoors store (REI,etc) or to any store with a camping section - target, walmart etc... and look for Emergency Blankets , sometimes called space blankets. They're usually a buck or 2. Silver on one side, usually gold on the other.
A nice can of spray adhesive or double sided tape and you can cover your grow space in no time.
Home dePot and the like also have mylar coated bubble wrap in the insulation section.
Peace