How to Look for Ancestors Bernard Goldman b ~1830 and Bessie Weintraub b 1832? #ukraine


I'm looking into the ancestors of Bernard Goldman born 1856 Russia, and his wife Bessie Weintraub born 1867 (supposedly in Belogorodka, Zaslav, Volhynia, Ukraine, but no one provided the source for that information).  
I found them both in the Iowa Census in 1895.
I've found 17 sources for him in the USA.  All my work is in FamilySearch (see LYWN-HDN Bernard Goldman).
I don't have any information on his parents. 
Bessie's parents were Abraham Weintraub and Ida Sara Roth (born about 1832 Pilkee, Russia -- but again unsourced).

My problem is, once the search moves beyond the USA (or England) I don't know how to look.  

How can I find some sources?
Thank you for your help.

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