Do you belong to a Church Group, or any other organization that could help you collect fabric? I'm just thinking that posting a notice at a Church or other organization could bring in donations that have been sitting on people's shelves unused.
Also, think about the social sites. Do you have a facebook you could use to "get the word out". Or any other social sites? Do family or friends of yours have one, and be willing to help you?
What about Craig's list on line? There is a "Want to Buy" section, and if you explain that the material is for homeless children and charity work, you could probably pick some up for free or very low cost. People will have bought fabric thinking they were going to make something, and it's still sitting there 3 years later, and they're bored with the idea. And you've just given them a charitable reason to get rid of it. Plus spell out what you need. If old blankets, certain kinds of clothes, sheets, etc., as well as fabric is needed, you'll get more.
I agree with the Good Will and Salvation Army. I've found a ton of fabric there for cheap, if you're not too fussy. For what I've used it for, it's been perfect.
Best Wishes to You!