I need to get a ±15 V power supply and I am wondering where to find it. Should I get a 30V DC power supply in its place?
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!
In a ±15V power supply, you can get 3 different voltages without the need of any external circuitry -15V , 0V and +15V. In a 30V power supply, you can get either 0V (ground by reference) or 30V. I'd suggest you get a ±15V power supply.
First of all you need to determine what current demand will be placed on the power supply. A ±15V supply is 2 X 15V supplies connected in series. Each supply will essentially be operating independently. It is unusual to find non-specialized power supplies with different current ratings for each rail. Depending on the load of whatever you’re connecting to it, the highest current requirement of either rail will determine the whole power supply rating. Some supplies are quoted as so many volts at X watts or Y amps. For power value multiply total voltage, 30V, by the required current to give power in watts.
A 30V power supply will not suffice if your application needs + and – supplies. Once you have determined the rating of the supply you can seek one out. I have used http://www.maplin.co.uk/