There is a simple and short answer to this question: Whatever they could get their hands on. In the history of guerrilla warfare, insurgents commonly drew on what was available to them or what they could improvise. This meant that in some cases, insurgents lacked arms and were underequipped until they found a solution, bought arms elsewhere or were provided with arms by a nation that supported their aims. The problem with using a wide array of different arms is that it puts a strain on already weak and threatened logistical lines.

To go in-depth about this, I'm going to draw from a previous answer of mine. The Viet Minh is a great example of how insurgents go from a state of acquiring whatever weapons they could get their hands on to being supplied by an external source as the insurgency progressed.

The Viet Minh had been collecting arms since its inception, but prior to 1945 there is very little information on where these small arms were acquired and any specifics about the arms themselves are only left to speculation. In one interesting case, General Vo Nguyen Giap himself had negotiated with some French officers in 1943 for, to quote Martin Windrow, "a couple of hundred old carbines with which to fight the Japanese with". Therefore, I will start at the earliest point in which we can identify the arms involved. Although it might seem ironic in hindsight, it was the United States that first armed the Viet Minh. In August 1945, the Office of Strategic Services (OSS) sent in the Deer mission, a team of OSS personnel who was responsible for training and preparing the Viet Minh to fight the Japanese. While the Viet Minh never got the chance to do so under OSS instructions, they were supplied with a limited amount of small arms together with other vital supplies. It was still not enough to supply an entire army. More weapons were acquired when the Japanese surrendered and an additional unknown amount of arms were acquired through Viet Minh purchases of arms from China.

The Viet Minh also set up their own weapons factories during this time but were unsuccessful in manufacturing bolt-action rifles. Copies of the British Sten submachine gun began to be manufactured in late 1946 and by 1948, the factories were beginning to manufacture bazookas and mortars. In general, the Viet Minh remained underequipped as the French Indochina War began in 1946 and as the Viet Minh was pushed out of the cities and into the countryside, their lack of weapons made them an easy target in the open. With the exception of captured weapons from French casualties, this would remain to be situation up until the great turning point of the French Indochina War: Chinese support for the Viet Minh.

With the creation of the People's Republic of China in 1949, the fruitful friendship and support that Ho Chi Minh had acquired in China throughout the years paid off as the decisions was made for the nascent republic to support Viet Minh's war against the French. Between April and September 1950, the Chinese provided the Viet Minh with over 14,000 rifles, 1,700 machine guns and mortars and anti-tank weapons numbering in the hundreds. This would only increase over the years leading up 1954 by which time the Viet Minh was fully equipped as a conventional army which included artillery and anti-aircraft guns. The arms from the Chinese were supplemented with large amount of French weapons that were captured during the spectacular Border campaign in late 1950.

But what sort of weapons were these? The captured French weapons consisted of the standard French rifle at the time, the MAS36 as well the MAT-49 submachine gun and the FM 24/29 light machine gun. There were also small quantites of the Browning Automatic Rifle (BAR) and the British Bren, all commonly captured from the Vietnamese National Army (VNA), the forerunner to the ARVN. From the Chinese, they received large quantities of Chinese and Czech Mauser bolt-action rifles as well as the Chinese copy of the Soviet PPSh-41, the Type 50. The Czech Zb26 and Zb30 was also provided to the Viet Minh by the Chinese. Smaller quantities of German weapons of Nazi origin was also captured, which had been used by the French who very much like the Viet Min had been desperate for weapons until 1950 when the US stepped in to support them.

During the late 1940s and the 1950s, a time when modern guerilla warfare was born, the way you acquired arms looked very much like this. You captured them, you were supplied with them, you made them or you already had them from the start. Some observations we can do from a wider range of insurgencies, ranging from Algeria to Cuba, is that they all pre-dominantly used WWII-era small arms. This isn't all too surprising considering the time period and the fact that this was the weapons that the opposite side also used (which in turn became weapons for the insurgents when captured and so forth). We sometimes see the question of what happened to all the arms that were left behind by armies during the Second World War and in some cases, they ended up in the hands of FLN guerilla in the djebel of Algeria or in the hands of a Viet Minh preparing to ambush French troops on RC 4. In the case of the insurgency in South Vietnam which began to pick up steam in the late 1950s, it's interesting to note that the Viet Cong mostly used the Soviet SKS until 1965, by which time the main force VC units were fully equipped with the AK-47.