Welcome to TBN. If you have an old glow plug you can read a number from it. Many tractors use NGK plugs. Even if yours aren't NGK you can contact them providing the make and model of your tractor and they can tell you which of their plugs is appropriate for it.
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.
If someone can give me directions I will post the Owners, Parts, and Service manuals, and the Spec Sheets for the T233 and the 273. I also have the LT200 Loader manual.