There have been a few forum threads on this subject in the past. Over three years ago I lost all hearing in the right ear and suffered impairment in the left overnight caused by a viral infection. I did some research on possible solutions and can offer some suggestions.
Possibly the most straight forward solution would be headphones originally designed for the DJ market with a stereo/mono switch. IIRC Beyer, Pioneer, Koss and possibly a few other manufacturers offer them. I intended to go with the Beyer, but they were in short supply at the time, so I started looking for other solutions.
Scansound offers a single stereo to mono earbud for single ear deafness users. I use one of these on a daily basis for TV viewing. It works well for that purpose, but I find it somewhat lacking for serious music listening.
Fairly simple stereo to mono converters can be made with resistors and some wire if you or someone you know is comfortable reading simple circuit diagrams and using a soldering iron. Circuit diagrams are readily found on the internet with a Google search. These converters enable the use of regular stereo headphones or earbuds. The inexpensive stereo to mono plugs sold by Radio Shack and others do not work in this application and can cause damage to some sound equipment.
Some older (and maybe some modern) sound equipment had stereo/mono switches built-in. Mono recordings were still in common use back in those ancient times! I was fortunate to have kept a Carver C-1 Preamp with a stereo/mono switch and headphone jack for all those years. I dusted it off and connected it to the MacBook Pro via an audioengine DAC feeding a pair of modded Fostex T50RP's. (I modded only the left side of the Fostex's, but left the right side in place.) This set-up gives me the best listening experience I've had since my hearing loss. Good luck on you quest! Vic
Edited by vlcakc - 5/1/14 at 6:59pm