Hi,.
the best source is eBay, there are sometimes really incredible offers. With a little luck, the price in a reasonable frame remains, but on some days the people even at the smallest offer konjac dumb and stupid. Best worldwide search for Amorphophallus and add this search to the Favoiten, then you will receive an email with the latest offers every day.
Otherwise is in Europe rather badly out specifically in Germany I know only the following shops:
http://www.samenladen.de > - range recommended, has also from time to time A. gigas, A. titanum and other rarities in the shop. The prices are adapted to the demand, I think because no one can complain.
http://www.seerosenwelt.de -> - has indeed just A. napalensis and you get two bulbifer clones in the offer, but ordinary tubers at an unbeatable price. Recommended also the many species of Arisaema and Tacca and Cardiocrinum.
http://www.rareplants.de -> - has a huge offer, but a real bargain but you will not find there normally. Partially purchased tubers at triple cost price in the shop are DVDs on eBay.
The really interesting shops sit most of the time in America, Asia or Australia. Since you have left but some loose change...
MfG Philipp