Proof of income is from your W-2's and Form 1099's that you received from your employer and financial institutions.
A tax return is not proof of income.
If you cannot obtain the W-2 from your employer, then for a fee you can get a complete copy of a W-2 from the Social Security Administration - https://faq.ssa.gov/en-us/Topic/article/KA-02501
You can get a free Wage and Income transcript from the IRS - https://www.irs.gov/individuals/get-transcript
