The address that you are supposed to send the 15103 is printed on the back of the form. The IRS says, "Complete both sides of this form and send it to us in the enclosed envelope. Be sure our address shows through the window."
This means that there may not be a single address to mail to, because this may be handled by various IRS offices, each with their own address.
Did you receive a CP59 notice that prompted you to complete the 15103? If so, please call the IRS at the toll-free number on the top right-hand corner of your notice. This will save you from having to call the main IRS number.