Best bet is to look for stuff on sale and percent off coupons.

When WalMart carried fabrics, they used to donate fabrics for our church's charity sewing. They also donate clothing for our clothing drive.

When I make charity quilts, I look at thrift stores for good used sheets. Also around here we have a website called Free Cycle, ( A Recycling Program) I can post that I am in need of fabric for charity and people will email me back if they want to get rid of some. All Free. Good Luck

A little off topic but since this is tax season I wanted to remind everyone that quilts you donate to charity can be deducted from your income tax. You can deduct the cost of the material but not the labor.

I used to go to garage sales and buy sheets for backing. I've got enough now. My LSQ donates batting and space one day a month to a group of ladies who will quilt my tops and distribute them to charity.

have you ever thought about visiting your local hotels and motels they throw a lot of sheets and bed covers out evry month they can not use sheets with any type of hole in it they have to pass a corperate inspection and if the inspector finds a hole in any of the bedding the get a bad mark for it so check there they maybe more than happy to donate them

Sassy Scrubs donates a lot of their scrap fabrics to different groups for use. Not sure if they would work with individuals, or if it would be worth the cost to have it shipped, but you could contact them to see. Or, think about that kind of business that might be closer to you. May not be typical quilting fabrics, but would work for the need.

I was just at Homespun Hearth online. Sometimes it has leftover bolts at wholesale prices.

make quilts for dog rescue, get the backings often from car boot sales / charity shops or markets, and have used felt at a cheap price for batting.

I use left over material from my quilts for the charity quilts I make, sometimes people who know I make charity quilts will also stop by with fabric and donate this towards the quilting. On occasion when I am asked what I am working on while at my craft or church groups, the next time I go in I will be given fabric to go towards that as well.
On the whole though I tend to make and donate more quilts that I have born the full cost for and look at this as being my gift to what ever cause.