Peats in Dublin 1 do a jack of the type you require. I assume you are simply transferring the sound via the earphone socket on the laptop to an input on your amplifier.
If the hifi has phono sockets then its a simple 3.5 to phono lead, everywhere sells them. Maplins or Peats etc. Cheaper on line probably. Try tower records too. Or it might be a 3.5 to 3.5 lead.
fyi the Apple Airport Express (i.e. their wirless router) has a speaker / headphone jack so you can connect the router to the stereo via the AUX connection.
You than then connect via wireless to the router to play iTunes over the steros speakers without being physically to the stereo.
Not so in my case, have connected up a few laptops and PC's to my amp. Have connected using the 3.5mm headphone outlet on the laptop or if your amp supports optical in you could buy a USB optical sound card and connect through that.
Turtle Beach Audio Advantage range would be suitable