The previous suggestions are all good. Another good source is Janet Eagleton. She and her son have a workshop and a small store in Perth, Scotland. Her website is not state of the art and the photos on it don't do her sporran's justice but she's a pleasure to work with. She has some very nice basic sporrans or can do a wide range of custom work as well.
Her site: www.scottish-sporrans.co.uk
Her email: scottishsporrans@btconnect.com

The Lewis Hose available through the Scottish Tartan Museum gift shop are great with a good selection of colors. They have a selection of flashes as well, but check out their "Traditional Garter Ties" as well.