I need a second (spare) smart key for my 2005 Mercedes A Class. Short of going through a Mercedes dealer is there a relatively cheap way of acquiring one?
No, there is no relatively cheap way. There ARE ways, but they're definitely not cheap. My car has a similar key to yours and the best price I was quoted was ZAR 1600, which translates to 100 Pounds or $160.
You may get away with having a spare key made without the electronic transponder. That should be much cheaper, but you won't be able to remote lock or cancel your alarm.