I can't find any universal adapters that to 3.3V. I bought a crappy £10 universal adapter from Maplins today, claiming to be "1000mA Unregulated", and it *sort of* works. The screen is incredibly flickery, far worse than anything I've seen when running on nearly-dead batteries, and a few times now the initial dark grey GP2X screen has shown up as a flickering mess (although the unit then goes on to boot & work normally).
The adapter should be outputting plenty of current, although I haven't tested this. The problem is, if anything, either low voltage, or fluctuating voltage (due to the "unregulated" adapter). I did briefly try upping the voltage, but all I got when I turned my gp2x on was a bright white screen for an instant, then it turned itself off again - I was so terrified I'd bricked it! However, after I put the voltage back down, luckily the unit came straight back on as normal. Such behaviour is dangerous and irresponsible, I know, but I thought it'd be safe since the unit had all the symptoms of being underpowered, and I wasn't even upping it to as high as 4v (assuming the dial on this adapter is a standard pot - that's probably where the danger lies
).
I'm going to be cheeky and take the adapter back, claim it wasn't the correct type - luckily it just came in a cardboard box, no seals to break.
I already have some rechargeable batteries, but they don't last very long in the unit (only 1300mAh); I can afford to wait until the official adapters are released. I'd recommend that anyone who has any kind of doubts should do the same.
I bought the adapter after someone posted in another thread that it had helped them to upgrade the firmware on units that wouldn't flash when running on batteries - I can safely say that if you get flickering and other odd behaviour out of your adapter, it won't help in the slightest when it comes to flashing.