There are a variety of books that cover the general issues of the Uprising.
The Sioux were armed w/ a myriad of then modern weapons. Bows, lances, war clubs, coup sticks and tomahawks were plentiful as well. There were a few modern Sharps carbines that came from God knows where in the mix as well. The Hawkens plains rifle was quite plentiful as were a variety of trade guns of various quality. There were even some flintlocks in use by the Sioux. Needless to say the Sioux were not uniformily well armed.
The Lakota Sioux, like all of the Plains Indians, favored close quarters fighting and would more often than not modify their weapons by shortening them to a manageable size for use on horseback. They also did not generally understand the need for the proper ammunition... which means a .58 minnie ball was just as effective as a .69 round ball or a handful of round river stones... They also often made their own ammo... so one cartridge may have 60 grains of Black Powder, the next 120... Maintenance of firearms was VERY erratic as some had no idea how to properly clean their firearms.
There was at least one English trader selling rifles to the Sioux... which accounted for quite a few 2 band Enfields in their ranks when they faced off against the Volunteer Army... At the same time however there were French traders trading rifles for furs at the same time. And of coarse the American traders who traded and sold guns as well. Also add in the mix of weapons looted from farmsteads that they had raided. Also the Company worth of 1842 Springfields taken from dead soldiers that were ambushed early in the campaign should be added to the mix.
I don't think there was ever any evidence that any traders were working under the auspices of any govt. but it could be quite possible, especially as some traders were working under the direct sponsership of their respective govt.
The Sioux were great fighting men and excellent horsemen, though they fought as individuals with the intent to further their personal glory and honor. They lacked the knowledge of massed fire or any kind of unit cohesion. At Ft Ridgley were faced with an immobile target that had the benefit of a battery of Cannon. They had never experianced Cannon before and quite frankly it scared the hell out of them.
I can't give you any specific sources off the top of my head but I would do a net search for the 3rd MN
Inf as they were the principle unit on hand fighting the Sioux. You might also try the MN historical societty web site as the uprising was quite significannt. There is a very real chance that some Indians were better armed than the soldiers as many were armed w/ low quality Belgian Rifles, at least one company carried shotguns and squirrel rifles that were scoured from Gun shops in the St Paul area at the beginning of the campaign.
Good luck I hope this helps somewhat.