Any small, 3/4-16 threaded filter will suffice just fine.
Those Kohler filters are nothing to get excited about; anything that fits will more than suffice.
It's not like the Kohler will screech to a halt because you used some alternate filter, as long as the attributes are "same-as or better" (flow, delta-P, etc).
The 51348 Wix (NG1348) will do find and can be easily found. Would be equivilant to the MC FL910S. If you have the room, the ubiquitous 51516; ph3600; FL400S; L20195 series would also work, and is on about every part shelf known to man, so price and availability should be a no-brainer.