Is it a mixing valve cartridge model?
Went through the same thing last week with my DELTA single handle shower valve. The $3 fix is what you did in replacing o rings and seals. If it still leaks, then it is time for the $40 fix and replace the whole inner cartridge.
I found my DELTA cartridge at the local hardware, surely you can find what you need there for your KOHLER.
Best if you have the model number with you, LOWES has a catalogue on site where you can match part numbers. Try this web URL.
Kohler: mixing valve : Search results