The tournaments come in the same order as the goods are listed in your vault or the relics in your Main Hall:
Marble, Steel, Planks, Crystal, Scrolls, Silk, Elixir, Dust, Gems. So next week is Elixir, followed by Dust
Also, in case you haven't noticed, the primary goods you need to negotiate are at the same position within the tier (1, 2, 3). So for the Elixir Tournament you'll need mostly Elixir, Marble, and Crystal for negotiating.