Your W-2: If you've misplaced your W-2, you can request another copy from your employer. Contact your human resources department now and tell them you need another copy of the W-2. You may have to fill out a form, but it shouldn't be too much of a hassle. If you haven't received your W-2 after reminding your employer, contact the IRS and have them send your company a prompt. You can do so by calling the IRS at 1-800-829-1040 and telling them about your situation. If you still can't get obtain your W-2, use the IRS Form 4852 as a substitute for it and refer back to your last pay stubs to help you fill out the information.
Last year's tax return: Can't find the previous year's tax return? Request a copy of your tax return from the IRS by filing a Form 4506-t, using the online system called Order a Transcript, or calling the IRS.
Form 1099: If you've been paid out more than $600 from an entity, you should have received a 1099 form from them. Banks will also send you a 1099 form if you've earned over $10 in interest. You should be able to download the form from your bank's website if you didn't receive one in the mail. If you can't find one online, you can see if a branch near you can issue you a copy.
Form 1098-T and 1098-E: Students, if you've misplaced your 1098-T forms or the Tuition Statement, check with your school to see if you can download another copy. Many of them will have a copy of your 1098-T online. As for the 1098-E or the Student Loan Interest Statement, check with your lender to see where your tax information was sent. If you paid less than $600 for interest, you will not be sent a 1098-E.
Receipts: If a receipt was not issued to you or if you can't find it, check to see if you can use your bank and credit card statements to prove the expense. The statement should show important details like the recipient's name, the date, and the amount. If you can't find a receipt, you can reach out to the establishment to see if they are willing to issue you another receipt.
Source: Flickr User Dave Dugdale