A Google search for "power supply kit" gets quite a few hits. These look OK;http://www.circuitspecialists.com/powsupkit
but there are several others.
These are just regulator boards; they require a transformer. A plug-pack is actually quite useful; it removes heat, bulk and weight from the device it powers.
I built a "plug-pack" for one of my projects for just those reasons;http://i46.tinypic.com/23iapzn.jpghttp://i45.tinypic.com/2qxuc9i.jpg
Don't dismiss the humble plug-pack.
An AC plug-pack would drive a regulator PCB like the ones in the link above but note my later comments about voltage.
You should read about/Google for information on LEDs, particularly driving multiple LEDs.
LEDs are not usually driven in parallel; small differences between them mean that different currents flow in each one.
They can be paralleled (I have done it) but it's not good practice and can blow the LEDs that "hog" the current.
Series connection is usual; that means that a supply voltage greater than the sum of the LED drops is required. Say six in series, each with a 2v drop; that's 12v @ 20mA.
(LED voltage drop depends on the colour and varies over a range; I used 2v for simplicity. 20mA is the usual current for ordinary LEDs).
To drive those six series LEDs through a current-limiting resistor, about 15v would be required; that gives a resistor value of 150 ohms.
To use 20 to 30 LEDs as you propose you could use a series/parallel circuit; series connection of, say 5 LEDs minimises the current-hogging problem so four parallel strings of five LEDs each string would work for 20 LEDs.
Each parallel string should have its own resistor; in the above example, a 15v supply with a 250 ohm resistor in each series string would work.
(Hope my maths is correct; I just did it in my head).
Another thing to consider; you may not need a regulated supply, just a single diode or a bridge with a transformer/plug-pack and a capacitor.
Millions of LED garden/path lights and similar lights work this way; it's much cheaper than a regulated supply and just as effective for non-precision applications.