Here comes moar CPSIA dumps, as of March 6th 2015! Now, all of the Wave 4 has been out of production.
Bowser - 4 months of production
Wario, Lucina, Robin and Pac-Man - 2 months of production
Super Mario - 1 month of production
The extended production length for Bowser is most likely why we're bound to find a wall of Bowsers for quite some time.
Wario and Pac-Man are relatively easy to produce and had a good amount of merchandise prior to amiibo, so they shouldn't be too difficult to find when the next wave will launch. Lucina and Robin might pose a lot of challenge, but they're nowhere near as impossible as Ness probably will.
If you're surprised on how many Super Mario amiibo are produced, it's probably because he's only found on the limited retail version of Mario Party 10. Though, the production might be more likely to continue because he's the Nintendo mascot and isn't too difficult to produce.
Source:
http://en-americas-support.nintendo.com/app/answers/detail/a_id/11021