I've found a couple of 110V USB chargers. The first one had an issue that it would start fine, then shut off when it deemed that no charge was necessary... which was bad when say trickle charging a phone, or some devices that had small batteries/draw.
The second one didn't have that issue. I haven't been using it as much lately... so I'll hunt it down later today.
Ahhh... wait, do you use 220V? There should be some direct wall plug-in types. But, a transformer charger might be easier.