Hi again,
The relay mentioned above is an addition and does not exist in your tractor at the moment, so you will not find it. If you check that thread referenced above you will find a mention of the place where I bought my low-voltage relay. It might be called Hartville tools or something similar.
But before you buy anything you probably do want to verify that the problem you're having IS with contacts in the safety circuit. I'm in Japan right now and cannot look at my tractor or my workshop manual. But if you look at your starter, I think you will see a small wire going to the starter solenoid. If there is only one wire going to the starter solenoid, then if you applied voltage from your battery to the terminal that wire goes to, it should activate the starter.
It would probably be best to have a switch in the wire between the battery terminal and the starter solenoid terminal, then you could apply and immediately remove voltage, and not have to deal with the spark you would likely get if you touched the wire directly to the starter solenoid terminal.
If you do this, you don't even have to start the engine, you only have to hear the starter start to turn it over to prove that your issues in the safety circuit contacts.
If you do try this, make sure the tractor is in neutral and that the parking brake is firmly set, just to be on the safe side.
Checking voltage with a test lamp can be confusing sometimes because there may be some voltage there, but not enough to cause the lamp to light. A voltmeter would be a better choice if you had one, but perhaps you don't.
I remember other threads here on TBN that address this issue and have specifics about the interposing relay, including part numbers and possible sources. The one I got was for John Deere and required me to splice a couple wires. I have read there is one specific for Kubota that is plug and play, but that was not the one I used. Perhaps someone will read this read and give you specific details.
Best thing you can do at this point is confirm that the problem is in the safety circuit contacts. Are you getting the "click click" sound when you try to start the engine? If so chances are pretty good that It is voltage drop within the safety circuit that's your problem. If there is no click click sound, then the problem could still be in the safety circuit, but perhaps a broken wire or a failed contact. If this is the case then the relay mentioned would not fix your problem.
Hope this helps!