Best bet is ATM. There are a couple next to the first luggage carousel you see after Immigration at DPS.
Once you surrender your custom form and turn left, you will see an endless window of foreign exchange agents, all offering same (bad) rate.
If you must change at local agencies, please make sure you bring crisp, unfolded (practically brand new) USD notes with no tear, stain or marks, not issued in 1996, and no 'H' or 'K' series. Supposedly this is the rules from Indonesian banks.
Enjoy Bali!