Supercard products can only be found online with the exception of maybe a handful of brick and mortar stores around the world. Personally speaking I don't know of any place other than online that sells Supercards, so here's a list of a lot of good, reliable online dealers for Supercard products.

USA
ModMyDS.com (http://www.modmyds.com )
Real Hot Stuff (http://www.realhotstuff.com )
Jandaman Import Shop (http://www.jandaman.com )
Modchip Man (http://www.modchipman.com )

CANADA
Kick Trading (http://www.kicktrading.ca )
GamerSection (http://www.gamersection.ca )

UNITED KINGDOM
Cheap Mod (http://www.cheapmod.com )
Supercard Store (http://www.supercardstore.com )
Console Plus (http://www.consoleplus.co.uk )

NOTE: I have only verified the US stores - all other stores are unverified and should be approached with caution!!!!

For all other areas of the world, please check out the official Supercard dealer page