Not sure how much success you have buying gift bags, but the majority of bags I come across tend to a) look a bit naff or b) cost a bit much.
I found this great tutorial which takes a piece of wrapping paper and transforms it into a tasteful gift bag in 3 simple steps. Great for leftover wrapping paper pieces or if you want the bag to follow along the same style lines as the gift inside. Or for those gifts which are hard to wrap and there isn't a bag the right size to be found.
Better still, why not purchase a large roll of plain brown paper and stamp your own design all over the paper before turning into a gift bag? A great project for the kids to do, especially for Easter and Christmas gifts or for their best friends birthday present.