I think you're referring to Archival Mist. I've seen it at Michaels, Meijer, and scrapbook stores. It's about $40/bottle (I bought mine w/a 50% off coupon at Michaels), but last a loooong time! I've literally had my bottle for 3 years, and it's not even close to being used up. It's great for including otherwise acid-y things in your books.
If you spray too much, it will make a white powdery layering on your item, but that wipes off with a clean photo cloth. Hope this helps!