To be able to sprout banana-seeds successfully, I would say you need atleast 50 or 100. I can see some of these companies have shocking prices for 5 or 10 seeds. Usually, these seeds are ordered from other online stores at wholesale prices. Not only monkey-fingers in this hobby, monkey-business too!
My favourites at the moment:
http://www.trebrown.com
http://www.kenibreedplants.com (directly from India, large orders only)
http://www.banana-tree.com (Best on price, quality on banana-seeds seems good)
http://www.rarepalmseeds.com (Poor service, but seeds I have recieved have been very good quality)
Erlend