One thing to do is look in the JewishGen Given Names database that might help with suggesting alternate "offical" names.
The more complicated answer is that depending on where you are searching for your ancestor's name, you might need to try different things. For example, if you are looking through ships' manifests, you have to keep in mind that someone from Poland who emigrated to the US around the turn of the 18th-19th century likely went through a German port of departure. Thus you should be looking for "Germanified" spellings of names. This Germanification might not affect the way Polish names are written as much at it would affect Russian names, but it's possible that your ancestor was a Russian speaker.
Some databases support Soundex indexing, so that may help match with inexact spelling; when that's not an option, try searching with just the strong consonants:
Fleker for example. In my experience, these tend to survive mispronunciation and mistranscription better than other letters.