Where can I find information about the founding members of the UN?

The following information comes from the UN Membership Research Guide.

Some founding members of the United Nations were not considered states at the time of the entry into force of the Charter, including the Belorussian Soviet Socialist Republic, India, the Philippine Commonwealth, and the Ukrainian Soviet Socialist Republic. Poland did not participate in the San Francisco Conference but was able to sign and ratify the Charter in 1945.

Oppenheim’s International Law: United Nations says, “although Article 3 speaks of the original members of the UN as ‘states’ that participated in the San Francisco Conference or signed the 1942 Declaration of the United Nations, not all the founding members were states as a matter of international law. The Soviet Republics of Byelorussia and Ukraine were invited to the San Francisco Conference and were founding members of the UN, although it is generally accepted that they were not states at the time. Similarly, India and the Philippines also participated at the San Francisco Conference and became founding members of the UN prior to their attainment of independence from the UK and the US, respectively”.

According to the The Charter of the United Nations: A  Commentary, “Although Art. 3 speaks of States as original members, the participants at the San Francisco Conference were aware that several of the participants were not ‘States’ in an international legal sense. This applies to the Soviet Republics of the Ukraine and Byelorussia, which became sovereign States only when the Soviet Union disintegrated at the end of 1991, and the Philippines and India, which only gained independence on 4 July 1946 and 15 August 1947, respectively”.

In The Law of the United Nations, Kelsen states, “According to the wording of Articles 3 and 4 only 'states' can be or become Members of the United Nations. However, among the communities which signed and ratified the Charter were some whose character as ‘states’ in the sense of international law was doubtful, since they had not the degree of independence from other states which is required by international law, thus, for instance, India, the Philippine Commonwealth, the Byelorussian, and the Ukrainian Soviet Socialist Republic”.

Poland did not attend the San Francisco Conference. According to the Repertory of Practice of United Nations Organs (1945-1954), Volume 1, Article 3, "At the time the Conference was convened, agreement had not been reached on recognition of a Polish Government by the Governments of the Sponsoring Powers (China, USSR, United Kingdom and United States)" (p. 164). On 28 June 1945, a government was announced; on 15 October 1945 Poland signed the Charter and on 24 October 1945 ratified the Charter, becoming one of the original members.

Additional resources:

A/C.1/212: First Committee : admission of new Members : letter, dated 8 October 1947, addressed to the Chairman of the 1st Committee / from the Chairman of the 6th Committee: 

A/CN.4/149 + Add.1 (3 December 1962): The succession of States in relation to membership in the United Nations: memorandum prepared by the Secretariat