Thankfully, there is far more surviving evidence of pre-Christian Slavic names (I saw that you mentioned in a similar post that you specifically sought pre-Christian names) than you assumed. The primary sources from the mediaeval-era Slavic polities record plenty of pre-Christian names. For example, the Pověst' Vremmenykh Let' (often referred to in English as the Primary Chronicle) of Kyivan Rus' contains many named characters (and not just the names of important historical figures) from before the adoption of Christianity as the state religion in 988.

However, in this quest of yours for "ancient Slavic names," I would heavily recommend narrowing your search, based on your needs. The pre-Christian period spanned thousands of years, many migrations, and uncountable cross-cultural exchanges (including the borrowing of many names), which eventually resulted in the formation of the Western, Eastern, and Southern branches of the Slavic languages. In addition, these pre-Christian names then underwent centuries of linguistic change into the present forms in each Slavic language.

Take, for example, the Old East Slavic name Володимѣръ (Volodiměrŭ in scholarly transliteration - pronounced either as "Volodimir" or "Volodimyer" depending on how one pronounces the letter ѣ). In Ukrainian, this name eventually developed into Володимир (Volodymyr). Meanwhile, in Belarusian, this name developed into the modern Уладзімір (Uladzimir). These two names descend from the same ancestor, yet they are distinct names today.

In conclusion, once you have narrowed your parameters and brushed up on your orthographic history, I would suggest looking in a good dictionary (even Wiktionary provides limited etymological information, depending on the entry) and/or delving into the primary sources. For I am more familiar with the sources of Kyivan Rus than for the other mediaeval Slavic polities, I will leave you with a citation to Harvard Ukrainian Research Institute's critical edition of the Pověst' Vremmenykh Let', one of the premier primary sources of Kyivan Rus'. This three-volume series provides a linear side-by-side comparison of all extant copies of the Pověst'. Perhaps your local university library will be able to obtain a copy for you.

Donald G. Ostrowski, Horace G. Lunt, and David J. Birnbaum, eds. The pověstʹ vremennykh leť: an interlinear collation and paradosis. Harvard Library of Early Ukrainian Literature: Texts 10*.* Cambridge: Harvard University Press, 2003.