All four prepositions can be used with the adjective 'commonplace'.
with: used to say that two or more people or things are doing something together or are involved in something.
to: used as a function word to indicate addition, attachment, connection, belonging, possession, accompaniment, or response (definition 4)
for: with respect to (definition 7)
That being said, there are some slight connotation differences between the four prepositions. 'Among' and 'with' are neutral sounding, while 'to' and 'for' are more forceful.
'Among' and 'with' would best be used when the statement is mostly true. (i.e. it's okay that some don't know.) 'To' is stronger, placing more emphasis that those who study law should know. 'For' is the strongest, particularly with the "soft" modifier 'should'. A professor may use the preposition 'for' when scolding a class for not learning a concept.
'For' is commonly used when there's a definite number or a statement of fact.
This knowledge should be commonplace for 80% of those who have training in law.
This knowledge is commonplace for those who have training in law.