I know what you are talking about. I have two of them from an old frag rack that I had made by a local reefer. I will post a pic of it in a minute. They are epoxy coated. I can't find them anywhere. They are not the disks. look for a magnet with the same pulling power and hopefully it will steer you in the right direction. I think these were in the neighborhood of 75lbs.
I'm sure they have little capsules, coin holders and stuff that you can encapsulate them and use acrylic/plastic bonders to truly seal them. Might be cheaper and work better then epoxy.
im pretty sure that ceramic magnets are reef safe and you can get them at home depot or lowes. research it first but thats what my rack is made with and its been in there a long time.