....you might look for headphones with an inline volume control. With luck, there'll be a set with balance.
If not (and you have some skill with a soldering iron) you could adjust balance on a set with a volume control by rewiring one channel so it's direct while leaving the other channel to be reduced by the volume control.
Alternatively add a resistor to one channel of a conventional headset -- explain your plans at a decent electronic components store and they'll sell you a range of resistors to experiment with.
Does any MP3 player have a balance control? I have VERY
unbalanced hearing due to an accident and need a 60 db
difference between the left and right chanels.
FYI MS's removing of the balance control from Vista has
REALLY pissed me off.