I built a replacement power supply for a Harman Kardon SoundStick by finding a laptop supply of approximately 35W on eBay. I bought an IBM Lenovo ThinkPad 16V 2.2A Laptop AC Power Supply Charger 85G6734 for £6.99 as it seemed about right and was a fair price (though then had to additionally by a clover style power chord for another £3 so check on eBay that you're buying the whole thing or that you have a power chord that is the same style).
I cut the end off of the power supply behind the choke as this was getting in the way.
Next fit a new plug - the Maplin 3 pin power DIN plug assembly (their code: N83KT £1.49) is the one you want. It's a bit complicated to assemble (and quite amazing for less than £2) but after a few tries it fits together well. There is some soldering required so you'll need to buy or borrow an iron and solder. I hadn't done any soldering for over 20 years - if you concentrate on melting the solder onto the joint to form a bond and don't worry about doing anything else you should be fine.
The pin connections are as shown here: http://www.bematech.com/support/downloads/power_connector_and_cable_specifications.pdf. On the Lenovo power supply the sheathed centre wire is the +ve (pin B) and the outer stranded wire the -ve (pin A).
Good luck.