I'm having a hard time picking between
These are one of the foundations of....
These are ones of the foundations of..
Or is there a more idiomatic way to express it?
I'm having a hard time picking, too, because of the "..." after the preposition. You've done yourself a bit of a disservice here, because there's not really enough context here to address the question with certainty!
...these are all valid ways to say this, too, but they avoid the very awkward “These are one” construct.
P.S. These are the kind of questions that would be better asked on ELL than ELU.
"This is one of the foundations . . . " or
"These are some of the foundations . . ."
"These one" is mismatched since "these" implies multiple and "one" is singular