Great to hear that you have been considered for a new position with a company and now just getting proof of employment is needed.
While contacting the IRS for your past W-2 statement can cost a bit and take some time, some other suggestions I can recommend:
1. Social Security can send you for a charge your past employment history. The time frame may not work for you in terms of how fast they can mail the information but worth looking into:
2. Background check run on your past history is something that can be done quickly and cost a bit less than the IRS or Social Security. Some services that are recommended are TalentShield, BeenVerified, CVCertify and Social Intelligence.
3. Another suggestion would be to contact previous employers (if they are still in business) and speak to HR or someone who can send a letter verifying that you worked with their firm and the years you were employed with them.