I'm sorry about your husband, that is a difficult time to go through, I know from experience.
Ask around, you might be surprised at how many people have stuff in their closets but don't want to have a yard sale or throw it out. Pass the word around family and friends.
And look for sheets in yard sales and thrift shops, they are most of what I'm using right now for lap robes for kids. Good for backing too, since they are so big. Most older quilts I've seen have white or creamy backing, muslin mostly but there's no law saying you can not use cotton sheets. Even a touch of polyester won't hurt that much to start with.