You can request an IP PIN through your IRS online account, in the IP PIN section of your profile page.
If know you already have an IP PIN and you lost the letter from the IRS (or never received it), you can get your IP PIN online at the IRS's Get an IP PIN site, or
call the IRS at 1-800-908-4490 to have your IP PIN mailed to you.
For more information, see: How do I get my 6-digit IP PIN?