The cheapest flash materials can be had at any craft store , but you have to improvise and have imagination.
I have used mylar packaging, mylar ribbons, and Christmas tinsel - in addition to the commercially available products. The krinkley and holographic types give off more flash ideal for muddy water conditions; however , for clear water conditions I try to use more subtle or 'fine' materials that create less flash and go for the flat mylar types. When tying for salt water , or pike flys , I generally use more and larger flash. The finest flash that I have (1/32") comes from a Christmas tassel from a craft store.
I have even 'disected' copperized scour balls (for cleaning dishes) for use on my beadhead 'copper johns'. The scouring balls are from a dollar store (4 / $1.oo ).

Dadflyer