I think, and this has nothing to do with volunteering opportunities, that if you want to do good then it is only up to you to do so. I mean there is so much need around. No need to travel, just look around you.
I feel volunteering involves some sort of need for a structure in situ, while actually it most of the time that the structure is badly missing. So creating one could be a good start, anywhere. It also involves wasting some energy on maintaining the structure, and structures we know tend to become rigid and, often, over time, inefficient (inefficient towards their original goals, efficient towards upkeeping the structure).
Also, I found that many Westerners actually only add up to the chaos if they go places and try (sure, in good faith) to help. The way I see things it takes a long, long and wise look before jumping into a help project
. So, I think, it makes good sense to me that plenty and extensive research
and prep is done, especially if you want to help in places that are far from your culture, far from your world.
So just these couple of ideas from me. I simply try to act when I see there is a need to act. So to say at times we end up as a 'one person self contained aid project' - maybe not a bad alternative to volunteering and so you can do much (hopefully) good wherever you happen to be.