Foodstuffs mostly, such as fungi and vermin. But other things as well, there is a reason why the sliver queen has a quadrouple treasure entry (that's four times standard!).
They also collect various metals that are used in the creation of the metallic slivers.
Also, they are quite excellent builders, although no one has still been able to decode their architecture, it's just to alien, with lots of pillars and strange carvings.
Later,