Not sure about platinum. I made most of my credits trading palladium bought from a station out on the edge of civilized space. I logged dozens of markets out that way. I'm at work today, but when I get home I'll check my notes, but pretty sure I've seen platinum for sale at one of them,
EDIT: My apologies, after searching thru 20 extraction and refinery systems, I could find no platinum for sell at any of them. Lots of good prices to buy it though.