Go to www.irs.gov.
Click on the “Get a tax transcript” link that is the third link under the “Tools” section.
That will get you to this page: https://www.irs.gov/Individuals/Get-Transcript
Now you need your Social Security Number, date of birth, and the address on your last return. Then click that big blue button, fill in your details, and you can pick a tax return going back to 2012. The IRS says they will send them to your address in 5-10 calendar days, but I guess if you have someone from your Senate office call they might FedEx them specially for you.
EDIT: As Old Good Old Left points out below, that big blue button will only get transcripts of the tax returns, not the actual tax returns themselves. However, on the same page, right above that big blue button, there is a link to Form 4506, which will allow you to request the actual tax returns, for up to 8 years, at $50 each. This will take up to 75 calendar days.
This will take about 10 minutes, so stop with the the-dog-ate-my-homework level excuses that you don’t have time to do it during the campaign, and stop being less open and honest about your taxes than even Carly Fiorina and Mitt Romney.
Of course, if you were really honest about being honest, you would have done this when you launched your campaign. Don’t just talk the talk Bernie, walk the walk as well.