It depends exactly what you are looking for. If you just want a single audio output, any mixer will do. Something affordable is the Behringer XENYX 1002B. It has five XLR inputs, and the output is just an RCA cable, which you can use a RCA to 3.5mm audio cable to your sound card's line input.
If you want to be able to have more control with the audio inside OBS, maybe have a look at the TASCAM US-16x08 or something similiar? With the US-16x08, you will need to use the ASIO plugin, but it's pretty simple to get up and running.