Ask your local builders where they get theirs from? Have a look-see locally at those properties that are being refurbished or self-builds ...... or ring your Council Offices?
Also, make sure that the over-all price is for drop off, situated in a safe site, chemicals, toilet rolls, as well as sucking out, flushing through and re-servicing prior to removal. I remember a local band paying over £20,000 for a line of serviced Porta-loos (which I think are a registered Company) ....... that was from the Thursday until the following Tuesday and the Company emptied and cleansed them several times a day.