With all due respect you must not be looking very hard. The biggest problem is sorting through what's available. Tractorhouse.com has 130 1770NT 16 row planters listed.  Fastline has 34.  John Deere Machine Finder has 182 1779NT's but not all are 16 row.  I even found a few on Craig's List.  Here's one from eastern Illinois

Something we found interesting a couple years ago, if you are willing to put up with individual boxes a 24 row 1770 could be bought a lot cheaper than a 16 central fill. A LOT cheaper.