How do I get a copy of a w2 from 1998 for free?

The only place is the employer who issued it. It may or may not be free at this point. They might even say they can't get it.
If you just need your gross income for '98, any Social Security office can tell you.

Call your employer and ask, but they may not have W2's going back that far. Why do you need a copy of a W2 that's 13 years old? If you're trying to prove income you can get a transcript of your 1998 tax return. See: https://www.askmehelpdesk.com/taxes/how-get-copy-old-tax-return-irs-318020.html

ebaines:

Correct me if I am wrong, but does not the IRS cut off the transcript availability at TEN years, which means the farthest back you can go is tax year 2000?

How to get an old w2 from 2010

For the 2010 W-2, your best bet is your employer.

If that is not an option, try the tax professional you used; he probably made a copy of the W-2.

Still no luck? Call the IRS and get a copy of your tax transcript for 2010. That will get you MOST of the W-2 information (no state data, though). For the state data, you need to contact the STATE tax department.