You will find a lot of online shops, which sell discs online.
I will just list a few here which have a incredibly wide range of discs, generally ultimate discs.
Amazon.com – discs from different sellers
The Wright Life – lots of disc, some collectable items
Internet Disc Shop – selling ultimate discs since 1994, some lovely designed team discs for sale
Discovering the world – lots of flying discs for all sports, own designs
UltimateDiscstore.com – cheap prices for discrafts, ultipro and Daredevil discs.
New Games – Frisbeesport – biggest selection of flying discs online in Germany, sell their own style of discs – eurodiscs for ultimate
Amazon.de – discs form different sellers