Hoye Tractor and Equipment has them listed on their web sight. I guess from what I see they are blank keys. I gave and just replace the whole switch for about 15 dollars
Any good locksmith will have the blanks or can order them. Good locksmith does not include your Walmart or similar copy key shop. And as posted, the dash key hole will accept almost any Napa etc. universal ignition set up.
To be a little more precise and since you didn't specify year and model. We have Yanmars from 1978 in YMs, F, and FX, and later and up to 1995 in Ke3 and F models. The keys are not the same and there at least 5 different key configurations but all are still available at a good key shop and key copies should at the most be $2 each.
It seems like all the YM1510 and bigger 3-cylinder YM's take the same JD key that LMTC described, the YM1401 is different, although I have had keys made for it at a local key shop. Some of the F and FX's take the JD key, but I have found the 3 digit F series takes a different key. Dave