What is the difference between a 30 V power supply and a ±15 V power supply?
A ±15V supply will have 3 connections (+15, -15, 0), whereas a 30V supply will only have two (+30, 0).

A 30V DC power supply probably isn't what you're looking for. You would need a way to provide a low-impedance mid-point at 15V to substitute for 'ground'-- assuming that it's isolated, or that you're very careful about how you interface it to external parts.

The best way to go is to get a ±15V supply, unless you really know what you're doing. Asking the question probably means you should just buy (or build) one!