I haven't used any mylar/tinsel before in white, but it is a good idea. At the time I first read your post, I couldn't think of anything off the top of my head, but later while I was searching for something unrelated I came across a Kreinik product (Kreinik Flash In A Tube™) that might work for you? The only issues are that it appears to be narrower (more along the lines of standard Flashabou) and the particular listing I found seems to be for a bulk amount ($21.95). If you don't want it all, I can buy half of it. Anyway, here is the link and the color is about half way down the page (100HL - White High Lustre). Good luck.
-Ken
http://frameofmind.kreinikmall.com/shops/product.php?productid=17270&cat=281&page=1