Alex,
A few points to consider:
1. As already mentioned, some of the latest chargers will work on a very wide range of supply voltage. If the label says something like "90 to 250V" then all you need is the physical plug adapter.
2. Alternatively, you could buy a suitable generic charger in the States. To go this route you need to check the specifications for the
output voltage and current of the charger and match them up.
3. Don't forget that all but the oldest of electrical systems in American homes supply 240V as well as 120V. You might have a 240V receptacle for a window air-cconditioner or something similar which you could use with an adapter lead.
4. If you want/need to go with a 120-to-240V transformer, several mail-order places can supply them. For example:
http://www.partsexpress.com/pe/showdetl.cfm?&DID=7&Partnumber=120-805(This one doesn't have a British socket on it, but you could use your plug adapter on the output.)