One work I'd highly recommend is A Morphology of New Testament Greek: A Review and Reference Grammar (Lanham and New York: University Press of America, 1994) by James A. Brooks and Carlton L. Winbery. You will have access to a number of morphological forms by using this grammar.
This reference work shows that IEREMIAS (nominative) would be IEREMIOU in the genitive singular. But IOUDAS would be IOUDA because some -AS nouns (masculine first declension) end with a genitive -OU, whereas others terminate with the Doric ending -A in the genitive singular. This phenomenon is what might be confusing at first--Brooks and Winbery provide helpful information on p. 49ff of their book.