How can I get a copy of an old tax return (2012) that my ex husband filed (we filed jointly that year)? He did the taxes and had the passwords.

Nope.  Since he's your "ex" you have no legal right to access his account. however, you can get a transcript for free from the IRS at <a rel="nofollow" target="_blank" href="http://www.irs.gov/individuals/get-transcript">http://www.irs.gov/individuals/get-transcript</a>. If you need a copy of the actual tax return, the IRS will charge you $50 for each year you need.

Well you could get a copy from him or the userID & password to access the return.         

You can get a return transcript from the IRS for free :  http://www.irs.gov/Individuals/Get-Transcript

For a $50 fee you can get a copy of the return from the IRS:  https://ttlc.intuit.com/questions/1901218-can-i-get-a-copy-of-my-tax-return-from-the-irs