We normally just buy the smallest single phase grundfos CRE (0.75kW I think?) with 3 or 4 stages, plumb it so that the discharge goes back to the tank, and use valving to provide backpressure so that sufficient water goes to the analysers.
It's probably not as energy efficient a solution as these small sample pumps, but replacement pumps are easy to source, and I've found the grundfos single phase motors on the CREs to be very reliable. I generally stay away from single phase pumps when i can avoid it but they are the exception. Since our reservoir sites are not often 3 phase supplied we are stuck with 1p.