Believe it or not, from YOU.
The IRS simply looks at your 1040 tax return and finds the information right there.
If you have written a check to the IRS in the last 3 or 4 years they have that as well.
The IRS goes through your tax returns and looks at your employers and the interest and dividend income information that you have given them.
They plug that information into the system and out pops a levy.
So, you the taxpayer give them the levy source information.
Also keep in mind that third-party supply IRS information about wages and 1099 information.
Third parties are required to send IRS information about all income.
So you plus the help of third parties give IRS the information to take money if you owe back taxes.