Welcome Calvin,
For building up for knots I will use strips of leather, electrical tape, masking tape, or athletic tape. I mostly use athletic tape. As for the belting if you get in touch with some of the sewing machine supply folks around here they would be able to get you fixed up with the belting you are looking for. The steel core that is talked about would be a steel cable most likely galvaized or stainless steel. For folks like myself who are striving to make gear that is functional and attractive as well as hold to "traditional" methods. I will always use some type of leather or rawhide as a core in any gear I make. Particularly in bosals. One reason would be that it would be a difficult to shape a cable core bosal and get it to hold the desired form, which is extremely important to the function of that peice of gear. There are lots of options when looking to make a core from rawhide or leather. The one you mentioned using a rounder, using individual strips or rawhide of leather bundled together and braided over. A strip of twisted rawhide, the list can go on. The difference in cores is to provide a desired feel. Soft and floppy or stiff and rigid. Hope this inspires some more questions, we all get a chance to learn when someone asks.
Rob