Dualing a fighter to a thief, eh? I would say level 6, but it depends. You'll spend a fair amount of time in thief-only form, but you'll be more powerful later. You can dual earlier if you want more time after dual, or later if you don't mind spending a long time in thief form. Kagain is very easy to find, early. Just go into buildings in cities early on.
Yeslick's [spoilers]
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In the 2nd or 3rd level of the Cloakwood Mines. [/spoilers]