If you like the 956, then look out for a box of 556 nibs. They come up periodically. It's the same nib, and especially if you can get a box from pre"Made in USA" days (I.e. Pre-1930) they are quite nice. I just picked up 100 for $35.
It's like the 914, the so-called Charles Schultz nib. The creator of the Peanuts comic strip would only use the Esterbrook 914 Radio nib. Now they go for ridiculous amounts because of that fact. Yet you can find a box of 14 Bank nibs for half or less. The earlier 14 Bank nibs are much better than even the 914. Same nib, but better grind and slightly more flexible.
The Esterbrooks to look for are in order from least to most desirable by imprint are
Esterbrook Made in USA (consistency of quality equivalent to today's nibs, i.e. spotty)
R. Esterbrook & Co. Made in U.S.A. (Good quality, but sometimes took shortcuts in coatings and often didn't grind anymore but instead stamped grooves)
R. Esterbrook & Co. (Starting to see some models get stamps, but otherwise still good. Grinds get more cursory on some models)
R. Esterbrook & Co's (great quality, from about 1897 to about 1925. Golden age of their mass production. Always great)
Esterbrook & Co or Esterbrook & CO (these are the rarest nibs because they are pre-1897, and the level of workmanship is the highest. This is when you regularly find double grinds and the greatest level of finish)
Find a box of 556's with "& Co's" and you'll be quite happy. And may have to pay $45.
Cat's out of the bag now. There go Esterbrook prices