There are numerous free websites that can assist in locating primary and secondary resources pertaining to the Spanish Autonomous Communities.
To link to the government websites of the Autonomous Communities via a map of Spain, see this link on the Spanish government's website. Individual links are listed below.
Note that the World Constitutions Illustrated database (available through HeinOnline by subscription) includes historical versions of the Spanish Constitution, as well as access to secondary sources that cover the topic of Spanish constitutional law.
Disputes involving the powers of the national government and the Autonomous Communities fall within the jurisdiction of the Constitutional Court. Some sections of the Court's website are available in English, and some opinions are translated into English.
Judicial power in Spain is unitary. There are superior courts in each of the Autonomous Communities that are part of the national judiciary. Information about each of these courts is posted on the website for Spain's judiciary under the "Tribunales Superiores de Justicia" tab. For an overview of the Spanish judiciary, see the section on "Judicial Power," Section 2.1.3 in GlobaLex's "UPDATE: Guide to Legal Research in Spain."
Spain's official government gazette, Boletín Oficial del Estado, is available electronically. Links to the official government gazettes for the Autonomous Communities, as well as the autonomous cities of Ceuta and Melilla, are listed below. All of these are published in Spanish, and some are also available in local languages--for example, Catalán, Galician, and Euskara.
This paper addresses the complex relationship between the Spanish Autonomous Communities and the EU.
There are numerous blogs that focus on international law, and several that focus on the law of Spain. Most of the latter are in Spanish. The following links provide lists of international and foreign law blogs. For a recent post on the Catalonian independence initiative, see the October 31, 2017 DipLawMatic Dialogues post, "Spain Fractured: Some Thoughts on the Catalonian Crisis" (Julienne E. Grant).