Decide what kind of porta-potty you need. The main porta-potty options include coin-operated toilet stalls, multistall set-ups, comfort stations (with hand-washing sinks included in the stalls), simple sani-flush (chemical flush) stalls, or small home or camping units.
Make a list of what kind of porta-potty is needed. Think about whether the unit will be hooked up to an existing sewer, septic, or holding tank. Also consider whether there will be a garden hose available or some other water source to which the porta-potty can be hooked up, if the porta-potty will be public, and if fees will be collected.
Choose from various portable toilet companies that sell porta-potties. When ordering, make sure that the company will be able to provide the quantity of porta-potties needed (if more than one is needed). Consider whether or not the company can deliver the porta-potties and if they are able to provide ongoing technical support. Don't assume that these things are a given; check them out first.
Check with the Better Business Bureau before placing an order to insure that the company has a good reputation.