Dougal
Sparkfun have a similar product and its 6A for the 5m length
(60mA/3 leds, 50mm spacing per 3 leds)
Some of the Laptop supplies are getting close, and I note that Sparkfun have 12v 100W (along with 150 and 300w) units available (at a price).
Most of the computer supplies have lots of 5v, but the 12v current is much lower.
A lot of the LCD monitors with external power are getting up to 100w, so you might get lucky.
Generally the LCD Monitors with a mains plug have a separate supply inside.
Be aware that you may want to replace the capacitors.
They generally have either poor quality or low hour rating capacitors for cost, but are easily replaced.
There is a good forum under
http://www.badcaps.net/ which cover a wide range of products/types.
Mark