It seems to me that fighting a war against the galactic empire is an expensive business. There needs to be a lot of hardware.
More over there are references to the fact that the Rebellion has money. They give Han a healthy reward, and they payed a lot for codes to access the second death star moon.
However I don't see much opportunity for them to make money. I can't see a 'Support the Rebellion' bake sale cutting it some how.
Is the financing of the Rebellion ever addressed in the literature?
The rebellion is made up of many organisations that suffered under the empire.
These include planetary governments in some cases, but more often factions with in the governments would provide assets and funding covertly. This enabled the governments to operate within the empire and against it to facilitate change.
Some organisations were deemed criminal and some were legitimately criminal provided funding to the rebellion. While the empire was focused on stamping out the rebellion they did not have as many resources to focus on the smuggling and crime going on in the empire. Ironically it was the same smuggling and criminal activity that allowed the rebellion to gather enough resources to pose a real threat.
Individuals and groups that dedicated their fortunes to the over throw of the Empire. Whether it was their ship, their money, or their skills a great many people committed treasure to the rebellion.
One of the biggest was Incom and Mon Calamari
The most notable equipment contributions included the defection of some Incom Corporation staff and all relevant material involved in the development of an advanced starfighter, the T-65 X-wing, which was acquired at the Battle of Fresia. With this advanced fighter, Rebel pilots had a clear advantage over the more numerous yet unshielded TIE fighters. With the help of Gial Ackbar, who had recently been freed by Rebel forces from slavery under the Empire, Mon Mothma then secured an alliance with the Mon Calamari, who openly joined the Rebel Alliance and used their renowned skill in ship construction to supply badly needed capital ships that could take on the Imperial Navy.
As well as out right piracy:
The Alliance would attack Imperial shipping in hit and run raids; both to disrupt Imperial supplies and operations, and to confiscate desperately needed materials. In these situations, their fighters had hyperdrive capability along prearranged routes which would allow this kind of harassment and escape before the Empire could react.
The Rebellion was started by dignitaries, namely Mon Mothma and Bail Organa, they were not exactly poor from the get-go.
Mon Mothma:
Mon Mothma was born in 48 BBY into a wealthy and influential family living on Chandrila.
And as I remember from reading the X-wing series, the forming of the Rebellion is mentioned, and her name is mention as the one who will supply the star fighters for the Rebellion I have forgotten which exact book it was and hence do not have a quote, however I found a blurb about it on the wiki:
During the early days of the Rebellion , Mothma took a leadership role in many important missions. These included gaining the X-wing starfighter for the Alliance and finding the plans for the first Death Star. Captain Raymus Antilles acted as her right hand man until his death.
(Both above exerts from http://starwars.wikia.com/wiki/Mon_Mothma)
Bail Prestor Organa
Bail was not born into wealth as Mothma was, however he did create a career in the senate for himself. He became ambassador for Alderaan, and later went on to marry the Queen Breha Organa, making him king and part of the royal family, and therefore ruler of Alderaan. He therefore had large sums of money, and even military, at his disposal. (See here for more info on Bail)
I also recall other people being mentioned in the X-wing novel (MonMothma for the starships, another man for the soldiers if I remember correctly...), if only I could remember which one it was and get a specific quote. If my memory returns I will edit this answer accordinly, but for now I think there is still sufficient evidence to support the fact that the Rebellion was created by people with great wealth, and could therefore afford to confront the Empire.
I think the answer is implied, if never said directly. Princess Leia Organa was, as her name suggests, a princess, and thus probably had access to some of her family's resources (even if it was only inherited money).
The Rebellion wasn't ever a tiny faction fighting underneath an empire, as much as it was another alliance of peoples fighting against a larger, domineering power. It's like imagining it to be the Germin Resistance fighting back against the Axis during World War II, when it is in fact more like the Allies.
See here for more of an explanation of how the Rebel alliance began.
There were many funding sources:
Sympathetic planets/governments (e.g. Alderaan)
Sympathetic individuals
Piracy (see raids on slavers in Han Solo trilogy)
