Hi Cat, it's a fact that soapsuds enemas usually cause more cramping than salt solution, but when you are constipated they also are more effective in relieving it.
If you have the time after a sse, it's recommended to take a salt solution enema to rinse out any soap residue that may remain.
In my practice and personally I'm using single use packets of Professional Disposables Inc (PDI) liquid castile soap. They come in boxes of 50 and you just mix one packet with a quart or two of water. It's a Potassium Glycerin based soap and mild enough to use for infants, during pregnancy, and in geriatric settings.
They cost about 10 cents per packet and you should be able to order them from any medical/surgical supply source.