The Baseball Cube has made an effort to compile uniform information for as many historical teams and seasons as possible. As a general rule on the site, blank values means the uniform number has not yet been researched while a dash (-) means the uniform # was not found.
Major League Uniform Numbers
This data is basically complete going back to the 1930s which is about as far back as the data seems to be readily available. The uniform numbers will be available on the Player Statistics pages and the team statistics pages.
Minor League Uniform Numbers
Minor League uniform data is a difficult challenge due to the fluidity of minor league rosters and the lack of an attempt by anyone to track this in the past. TBC has made an attempt using archived rosters and wikipedia to research minor league numbers as far back as 1977. The result is that numbers are heavily skewed towards the present day and towards players who spent significant time on a roster. Players in the 1970s and 1980s with uniform numbers were basically extracted from minor league baseball cards where their uniform number was visible. It is likely that at the time you are reading this, that not all minor league player seasons have been researched.
College Uniform Numbers
A much lower priority long-term project, College uniform numbers will likely eventually be filled at a rate of 99%. Most college rosters since 2002 are available through research tools and so we'll likely get them all included. At this time, most numbers skew towards current seasons.