Had a look about. Apparently the cable is just a standard USB to 2.5mm DC setup - which is easy to find for dirt cheap on eBay etc (around £1.29 posted). The problem is that most run at 5v, and the GBC runs at 3v, so the voltage is a bit higher. However, BennVenn's site says:
Note: GBC and MGB originally operate of 3volts though the flyback converter design of the internal power supply will operate up to 5.6volts safely. It will operate at a lower current and run cooler when operating off higher voltages.
I found that JellyBelly Customs has a 3.3v version they got made specially for £5 + £2.50 postage.
I've ended up getting one of the 5v cables and one of the JellyBelly cables and will see if I notice any difference. If I had discovered the 5v cheapo cables before ordering the JB one I'd probably just have chanced my arm with that, given the cheaper price.