Code For 2001 Galant Radio

I Had To Have Oxygen Sensor Replaced, And Had To Unhook Battery. Now I Need A Code To Unlock Radio. How Do I Get
This, And Do I Have To Go To A Dealer?

I had just gotten a new battery on my galant and now it is asking me for a code . I know you get three chances to enter ,, i have one chance left.. if someone knows how , i would appreciate it.