There is also a store near Clarendon that sells it, and maybe the Scottish store in Alexandria.
One can also order by the case from
http://www.irn-bru-usa.com/index.html although I'm curious about this disclaimer...
The manufacturer, A.G.Barr plc of Scotland, formulated Irn-Bru for the US market to comply with FDA requirements for permissible ingredients and US food labeling. Irn-Bru is imported and supplied by Great Scot International. It comes in packs of 12 x 16oz 500ml) plastic bottles. {Note Irn-Bru is not a significant source of iron}