Hello Dianne, I saw your post awhile back, and was currious what respnces it may get. Well I see you aren't getting any responces. I'll try to help.
I don't know of any actual product made to be a coal bin. From reading this forum, most boal buring folks make a bin out of pressure treated lumber and use their creativity and ingenuity to make what they need for their particular location.
I would suggest going to a TSC, or other farm supplies store. The other option would be go to a farm tractor and equipement company and see what they have in used grain storage boxes.
There are steel boxes mounted on a trailer frame that farmers use to collect grain as it is harvested, and to store grain in and dispense the grain from a discharge door. But these are usually pretty big, a lot bigger I think that you want or need.
Do a search on this forum for coal bin, there were several threads with photos and good information last season.
Hope this helps, Greg L