The LRA, off course, had weapons when they entered the Garamba in 2005. They just did not launch any attack for a long time.
"From mid-2005, when the LRA began to prepare for the
peace process that was to become the Juba talks, its fighters and high command moved towards the border of the DRC in Western Equatoria. On their way west, some of the fiercest fighting with the SPLA occurred in areas that had previously been untouched by the LRA, such as the road between Yei and Juba (BBC News, 2005). However, the LRA has denied that these attacks were carried out by their troops"

The Karthum supply actually must have stopped (temporarily) for logistical reasons while the LRA was in the Garamba. "While civilian and military interviewees say that Khartoum supplied equipment to the Ugandan rebels well into 2006, the LRA says that such supplies ceased long before then.36 Others have reported that supplies stopped in November 2005, when the LRA crossed into Garamba National Park in DRC."

The surge in firepower actually comes from a different "timing". The LRA, as a tactic, is used to leave arm caches in strategic areas. With the Karthum supplied weapons in the past, it was easy to build multiple caches in Southern Sudan and North Congo and to reuse those weapons as soon as they were hunted down.

I don't think they bought those weapons from the FARDC as the FARDC presence in the area was small during a long time and many other weapon salors were available (Mbororos, Equatorian, GoSS corrupt, Ituri militias,...). The LRA never worked with a lot of cash (they are out of the monetary system) and more by extorsion or support.

Quotes from the very interesting @small arms survey report@ http://www.smallarmssurvey.org/files...%208%20LRA.pdf