http://www.shop01media.com/
You could buy from here too, however they are not my 100% failsafe favourite so there is a chance to get a clone
http://www.spielite.de/24-nintendo-3ds-flashkarte
http://www.elewelt.com/
Other than this I don't really know many resellers.
I would suggest to wait a bit until shop01media has them in stock again, they normally ship either from Poland or Sweden, whereas the other two are the same and ship from hongkong or shanghai to germany, then to you. :/
I just put in gateway uk and this here popped up but no warranty if this is real or another fake reseller:
http://www.3ds-gateway.co.uk/
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