Hi: Good question. I'm not sure about a "stiff"; but I think that these products fall into the general classification of Mylar.
The hobby stores have it in several forms for cheap, under the name
"Gift Shred" in many different colors. I've even seen an even dirt cheaper form in the Dollar Stores, but only in the Pearlescent or "Opaline" color.
When you write "stiff" I think you must be referring to a thicker form?
I found out that Mylar, in sheets, is becoming rare, because there are health issues with its production, so the costs are going up. I am not sure exactly if Mylar is the only product that approximates a KrystalFlash, but perhaps someone else can share their knowledge and benefit us both.