Rims are measured as bead width x diameter x bolt loops.
Bead width is usually about 1 inch less than the lip to lip width of the rim. Diameter is the tire mounting diameter - in your case, 24.
Bolt loops are the number of bolt attachments for mounting the rim to the dish.
You can go on ebay and search for the rim using, for instance, "12x24x6 rim". Somewhere in their listings should be a picture or drawing of your rim. Probably be in the $250 - $300 range.
Good luck - let us know how you do.