This is a small list of commonly used shops and online stores that we use.
White Water The Canoe Centre Shepperton
Lomo Watersport and Ewetsuits UK - Online store - wetsuits and drysuits and other paddling kit. Cheaper than shops but make sure you order correct size.
Reed chillcheater - Online store.
UKRGB Private Sales - Popular Forum with Private Sales of second hand kayaks and kit.
EBAY - Good place to find second hand kayaks and kit.
Canoe Polo Zone - Buy canoe polo gear including Streamlyte polo paddles
DBDesigns - Buy canoe polo gear