This site gives apportionment data using the 2000 census.
https://www.census.gov/topics/public-sector/congressional-apportionment.html
You can find the reference to Article 1, Section 2 of the United States Constitution (among many other interesting things) at this site:
http://www.law.cornell.edu/constitution/constitution.articlei.html
Data from the first federal congress can be found here:
http://www.gwu.edu/~ffcp/exhibit/p14/