Hey Everyone, need help with something. Did a search, but couldn't find anything.

I've been having a lot of trouble with my Shimano BR-RS785 brakes on my GT Grade 105 Carbon. They are fine at stopping, but when I get down to about 5mph, they start screeching like a banshee. I've tried cleaning the pads and rotors, tried sanding down the pads, everything. I'm to the point where I think that maybe the pads are contaminated and I need to try new pads. Finding pads for these are not easy though. The pads that came with it are J04C pads, but I'm not really finding the resin equivalent of them online anywhere. I also don't know that I necessarily need the cooling fins as I live in a fairly flat area and don't have any long descents. Does anyone know what pads will work for these? Anyone having similar issues with squealing? Any help is appreciated!