You need a Masters degree in order to apply, but it need not be in SLP. It depends on what you expect to do with the degree. If you want to operate as a Speech Language Pathologist you need to complete a SLP Master's Degree first. If you are interested only in research you don't need to complete a SLP Masters Degree. It will be necessary to take some Master of Arts courses in order to do research in the area or eventually teach in the area.
There is a graduate assistantship program availabe, for which students must apply. In addition, your application packet lists several different scholarships and grant programs which you may apply to.