Hi,
I spent some time searching for DD floppies to buy. New HD floppies are still easy to find (for the time being), but I reckon DD floppies would be more stable in DD use. And, because they are not easy to find, decided to share my finding here for those who don't have SD / MMC cart / HD cart / need to use floppies on their MSX's for whatever reason. This might save someones time. 
Of course, there's the www.floppydisk[s].com, which was mentioned elsewhere, too. But their floppies are quite pricey (double the price of new HD floppies).
Then there's amigakit (regional shops for US, EU, Canada, UK, so customs is no a problem).
They sell DD floppies (formatted for Amiga), at much cheaper and in smaller quantities! Still more expensive that HD, though. They also seem to sell competition Pro for Amiga, which of course would be quite trivial to convert to MSX - though you need to change the cord, because PiN 7 is probably not connected at all to the connector (changing the connector might be an issue for some MSX's because their joystick ports don't protrude at all, so the standard D-sub casings could be in the way).
Any other suggestions where to get floppies? Or joysticks for that matter =). Already ordered some pads as suggested by Nyyrikki in another thread =).
(same goes for tapes). 
. Also, I have all my MSX files / floppies transferred to a PC... so if a floppy dies I can always replace it 