Is it for your flashgun or studio strobes? Studio strobes I got them from Cathay Photo.
For my flashgun, got mine from this eBay store. I also requested to change the PlusGreen to another set of CTO since it'll be used more frequently, to me at least. I cut mine exactly to the size of 580EXII head and created extra flap to slip in to the wide-angle diffuser panel so it can sit in nicely without the need of any velcro. Am also be able to use sto-fen cup with the gel on.