Congrats on your new (to you) tractor and welcome to the forum!
I'd start by doing a google search. That tractor was made about 40 years ago so you're going to be looking for places that either have NOS (new old stock - basically a new/not-used item for an older piece of equipment) or are parting out/salvaging a tractor. I'm guessing you could probably find one for under $100.
You may also need to have your serial number handy. It's not unusual to see something related to a part in that age tractor that may say something like "fits JD 2010 models with S/N nnnnnn and higher (or lower)". They're always "tweaking" the tractors so if they find a bug in an item, they'll switch it out on the newer models.
Your S/N will be found on the right hand side of the engine block.