Airport counters
Kansai Airport has currency exchange counters in the arrivals area, useful for getting a small amount of yen in hand immediately after landing, though rates here are rarely the best available — fine for the taxi or first meal, not for exchanging a whole trip's budget. See getting to Osaka for the wider arrival routine.
Post offices
Japan Post branches offer currency exchange at many locations, generally with more competitive rates than airport counters, though hours are limited to standard weekday business hours and queues can be long at busier branches.
ATM withdrawals: often the simplest option
For most visitors, withdrawing cash directly from a 7-Eleven or Japan Post ATM with a card that doesn't charge foreign transaction fees ends up simpler and often cheaper than any dedicated exchange counter — see money basics for more on which cards work reliably.
A practical mix
A reasonable approach: exchange a small amount at the airport for immediate cash needs, then rely on ATM withdrawals for the rest of the trip rather than carrying or exchanging a large sum in cash all at once.