There's quite a wide range of volunteer opportunities on the internet. In fact, there seems to be so many organisations that there must be some opportunity of some description just waiting for you exploit.
Before you undertake any voluntary work, you will need to do some homework. Will your 'employer' provide you with free transport to your new loaction? How about medical cover? Will there be a US Embassy that can help you if you get intro trouble? Vaccinations? Travel Documents?
So, before you venture off to some place you may never have even heard of before, make sure that you will not be left out on a limb if complications arise. After all, you are giving your services free of charge, and while you expect nothing in return (but to get from A to B in one piece, and back again), you should expect that you will not be expected to go beyond what is expected of you.
With all of this in mind, we give you some official organisation links to get you started.
Still nothing that catches your eye? The US State Department is giving you lots of useful links, along with our own handful of suggestions. But if you are looking for something VERY specific, then there is always your favourite search engine. Be warned (again) though unless you properly verify any organisation you finsd on the internet, there can be no guarnatee that it will provide you with exactly what you seek.
Also be wary of any organisation that asks you for payment towards your expenses, especially if this is upfront and with no guarantee you are not being conned.
Good luck with your search.