Not that I make skins or anything at all, but when I modify my skins (ie, putting flags and numbers on them), I usually just go to Google images and type in "UK Flag" or whatever. Then crop it down in PS or whatever you're using.
What the Duck said ^^ Just grab the images from the internet and edit them with your prefered image editing application. Although I'm definitely no expert in skinning myself
Surely this should be in the skins section not gen chat?
Define "flags and stuff" then we can better help you. Do you mean country flags or chequered flags? by stuff do you mean numbers?patterns?stripes?sponsor logo's? etc.