For those who can still get their congregations to add a service this week:
Here is the 1945 Minhat Todah- Service for Thanksgiving Day, Congregation Shearith Israel, NY by Rabbi David de Sola Pool.
Happy Holiday- Let me know if anyone uses it.
Thanksgiving Service- 1945 Rabbi de Sola Pool (pdf of full service)
There are several good sermons from Rabbi Leo Jung for Thanksgiving Sabbath and a couple of shiurim on the web from Rav Soloveitchik from the Wednesday on the eve of Thanksgiving. Here is one of my favorites from Nov. 22, 1975
Update 2017
in 2016, I posted a 1905 service for the Sabbath before Thanksgiving written by Rev H. Pereira Mendes of the Spanish- Portuguese synagogue of NY
And in 2014, I posted the Thanksgiving service from Kehilath Jeshurun 1940 and prayer from Rabbi Joseph Lookstein.
The Book of Doctrines and Opinions: 
i would be interested in in finding out more about saying Tachunun on Thanksgiving
The Spanish Portuguese minhag is not to say it.
Professor Saul Lieberman did not say Tahanun on Thanksgiving.
Whereas the Spanish Genovese minhag is to omit Tachunun on Columbus Day.
This scan is missing page 5/6. Does anyone have a complete scan or know how to get a copy of the Seder?
There is a copy at most of the seminaries and major libraries. If anyone sends me the scanned page, I will put it up.
Oddly enough, there is not a copy at Shearith Israel or Mikveh Israel.
I used to daven at S&P on Thanksgiving morning when I was in HS and college, because they had a little breakfast, and then you could watch the parade from the porch of the shul (behind the gate, so you have to stand on chairs). Mr Goldfarb, Scoutmaster of Troop 613 [Manhattan], was the shamash of the S&P, so every year he invited the troop members to come and watch the parade.
They did not do this whole service. They did not say Tachanun, and they would add the last 4 chapters of Hallel (Hallel is Ps 114-118), recited as tehillim, not as Hallel, i.e., without the repetitions that make it Hallel, after the end of Shacharit. Rabbi Angel (Sr; Jeff was a school-child at the time) would give a little speech at breakfast about the S&P celebrating Thanksgiving since the very beginning, and Rev Gershom Mendes Seixas 40-page sermon for the first national Thanksgiving (1789).
One year they did say all of Hallel, but that year Thanksgiving fell on Rosh Chodesh.