Try ISA.ORG to locate an instrument vendor directory.
Look for Sensall or Kay-Ray/Sensall for ultrasonic level instruments. I found that their point level switches worked OK. I avoid point level these days prefering continuous level as you describe.
Many of us use RADAR instead of ultrasonic level instruments. Ohmart/Vega is competitive.
If it is a wastewater lift station you may want to look at a Radar unit they have dropped in price dramatically to a point where they are competitive with Ultrasonic. Wastewater lift stations have a tendency to have a layer of foam or grease and the Radar unit will outperform Ultrasonic in that environment. Siemens has a 2 wire unit LR-200 I believe is the model, I have a few mounted in tanks that have a layer of foam with no incident.