In the supply category you can set up the sources (exchanges) you want to use:
By default, your campaign will get impressions from all supply sources. You can also choose specific suppliers that you want to use by checking SSPs from the list.
We are constantly adding new partners and we recommend you keep the "Allow traffic from new sources" feature ON.
However, one thing to keep in mind is that there are some supply sources which may not always provide traffic due to its unavailability for certain campaigns thematic, ad formats, countries etc.