What are some good primary sources that make mention of the cults of ancient Rome. specifically the later periods, post Caesar, pre-Christianity. more specifically Sol Invictus and the cult of Mithras.
For Mithras, there is no better place to start than Roger Pearse's website.
There you will find not only introductory information, but a very well maintained catalogue of pictures related to inscriptions etc., of Mithras related material, as well as a comprehensive list of literary references to Mithras with English translation.