For the cheapest, bulk stuff, I look at Party City-a 'Party' store-costumes, Etc.
There is always cheap mylar to be found. You have to collect it as you find it in differant items. The last Great deal I found was in the form of pom-poms for (really cheesy) celebrations of some sort. For five bucks, I got 2 years worth of 12 inch strips of very durable stuff. I ve found that there are always bags of cheap 'kazoo type whistles' that have a mylar skirt. the skirts are only six inches long, but are fine for smaller lure.
Around christmas, you can get a lifetimes supply for less than 30 bucks in the form of tinsel-type dispays.