As a general rule, you need to use the same brand for transmitter and receiver. If you're using the Slash's stock receiver, you need a Traxxas transmitter to go with it. If you want to use the Futaba transmitter, you need to get a Futaba receiver to put in the vehicle.
Some brands have different protocols for air vs. surface products - keep that in mind when you're looking. Also, Futaba has several current protocols which are not all compatible with one another. Make sure the transmitter/receiver pair that you're wanting to use together will actually work together.
Andy