Zip I'm not sure on a 36 Volt System what size you need for a Single Breaker.
Most 24 Volt use a 50 Amp, I'm pretty sure.
The Old Rule Of Wiring was 8 Gauge for 12 Volt TM, can be used for 24 volt.
6 Gauge for 24 Volt is Preferred.
Not sure for a 36 Volt System, but I'd think it should be At Least 6 Gauge.
Since you just Bought the TM, did it come with a Owners Manual? If so there is probably a section in it that says what size Breaker to use.
Now where to get them. Places like US Boats and Others.
Here is a Link to a page that carries several styles:
Good Luck man and I hope this is the last bit for you. Really sucks when you get something New To You and the former owner didn't take care of it.