I bought mine from Eco-Reefer. I doubt you can get one from them anymore though. I know Eshopps makes a magnetic probe holder. You can buy it from jlaquatics or goreef.
I made a temp probe holder using a piece of blue rigid insulation. I just drilled some holes in it and let it float around my fuge area before I found a probe holder.
I seen some stainless ones at a michigan lfs. But yu can make one with a old mag float Gt some black acrylic and drill holes. Amd attach to the mag flot or go to the dollar store and buy some suction cups
I was thinking a mag float but I don't have an extra unless someone has one layinng around and I already ordered with brs I hate the last min things you think off AFTER you place orders!