eBay. You can search for “TYM 273 glow plug”, and get one listing for individual plugs at $19/ea. Or you can search for, “mitsubishi s3l2 glow plug”, and get sets of three for around $35 to $40.
The Mitsubishi S3L2, was used in a lot of different pieces of equipment, and brands of tractors. And, the parts related to the engine are the same, regardless of the piece of equipment the engine was installed in.