Whew, that's tough—especially in English! You might want to check in the work of Derek Fewster, who specializes on Finland but who might have other sources for you. Check out this article here (may require creating a login), although its short bibliography looks almost entirely Finnish. You might also want to see if Florin Curta has any useful bibliography for you in his Making of the Slavs, although his focus is on the emergence of a Slavic identity in SE Europe beginning perhaps as early as the 500s, so a bit outside your interests in both geography and time, but still immediately adjacent to them.

If you're at a university, I'd also recommend asking at your library. Almost all universities have research librarians who are paid (out of the students' tuition) to help generate research bibliographies. Most universities also subscribe to particular bibliographic services, such as through Oxford or Cambridge, which might be of further help. A librarian could walk you through how to use these. Public libraries might also be able to help, although they don't always have the same level of research focus. It seems like you already have the right terms (Roman or Early Iron Age, Baltic, north or northwest? Russia; perhaps also Bulgar or Avar?; Slav might not be useful, since there weren't yet 'Slavs' during this period, although some 'Slavic' histories might include reference to your prehistoric area of interest) which should help steer you on target.