You’re not mentioning where you are going, specifically. Websites like what you are searching for do exist, for some regions and locations, but focus on that region or location only.
So, where are you going?
Also, as has been pointed out, you’re typically much better off, looking for a volunteer position after you’ve arrived at your destination.
Particularly in less developed countries, assuming that’s where you are thinking of heading, there are typically plenty of options, though, for less developed countries, online resources will be scant.
But, it all depends on where you are going.
I might do my volunteering AFTER I arrived at my target city. And then walk in to the site of my choice.
The director of an institution has a choice of two volunteers, both of whom are foreign, and speaks English better than the local language: Someone who has called or emailed him or her, leaving a message, or someone who’s sitting in front of him/her, and (presumably) has made a good impression. One of these candidates probably has a much better chance than the other. Plus the fact that you will get to “interview” the director and institution as well.
I find googling often yields annoying companies that want to charge you to volunteer. However someone gave me this resource for South America that is amazingly detailed and covers just about every option or country you could think of in that area of the world. Hope it’s of interest / benefit:
Credit:stackoverflow.com‘