How do you buy a key for my Honda Harmony 1011 SA riding lawnmower?

Take out the ignition cylinder and take it to a competent locksmith. They'll take the ignition apart and be able to make a key from the tumbler positions inside. I took my Honda Shadow ignition to a guy because my friend had lost the keys and he had it ready in 1 day and low and behold, the keys worked perfectly.

Shouldn't cost over about $10-$15, $20 at the highest. Shop around.