Verified answer

You can probably call the IRS to get this information. However, if this is an individual proprietorship without employees calling your customers then they need no tax number, they simply use their SS number.

There are a variety of places to get that, but you are not legally allowed to file before you get your W-2. And if you use tax software and efile anyway, figuring they won't know you didn't have it, you take a chance of putting in some wrong numbers and having to file an amended return.

Being nosy is not a valid reason for you to need to know their tax id.

All EINs are treated as sensitive as SSNs.