Information

What do the slaad know of the cultists or the kobolds? Excellent question. The power of the volcano calls to the slaad, and draws them to its heart. Because of this, they frequently visit the feet of the volcano, searching for entrance, and probing every hole or crevice in an effort to find a way in.
They fear the dragon, but little else, and visit in the night to learn its secrets. They have discovered the entrance to the kobold lair, and begun gathering info about their coming and going, looking for a way to exploit it.
The cultists are further away, but many of the slaad passed near it following the call to the portal in their village. Their journey gave them a good view of the location (otherwise unknown), layout of the approach (including guard locations), and a sense of movement (including patrols). Their knowledge of it is passive, though, garnered from shared observation that is increasingly outdated. It should be enough to arouse the players' interest, and suggest they visit before going to the lair itself.
If the party fight the slaad instead of communicating with them, this information can be found in the green slaad chief's hut in the form of crude maps and sketchy info. He was gathering information for the giants, who had begun taking an interest in the movement of the dragon cult, and its connection with the kobolds serving the dragon, as well as the dragon itself.
There is also a cryptic reference to something at the cult location that is being prepared for the dragon. What that is can't be discovered from the slaad, but should encourage the party to explore further. There is no map of a trail leading out of the swamp, only a direction of travel for each location. Actual inspection outside of the village will reveal a path leading toward the swamp's edge, but more toward the mountain and the entrance to the kobold lair.