The part number for the mount is CB5-5311 and I did find one listing on eBay when I searched up the part number (though it is a bit pricey). I did more searching but didn't come up with any other results with the tiny bit of looking I did. Another thing that might help with searching for a replacement mount is that the Canon Rebel T100 is also known as the Canon 4000D and Canon 3000D, so you can try searching using those model names too.