Honestly spare battery is your best bet. Radio shack (US) & Maplins (UK) sell all kinds of chargers but they'll not be as good as second battery or mains charger.
Solar needs to be big or they'll be too slow; I think one of the best I've seen is the Powermonkey range which some have solar panels, some with chargeable battery, some with both (I think the solar charges it's own battery for you to use later, & you can charge it on the mains too). These will be considerably larger & more expensive than a keyfob option.
The shops mentioned above usually sell keyfob chargers such as AAA or AA battery powered ones, but they're only good for emergency charge for a few calls.