Generally speaking, a standard 5050 LED strip contains a number of individual LEDs soldered to a flexible circuit board (strip). Each of the physical LEDs is actually 3 LED dies, one each for red, green, and blue. Typically these LEDs draw 20mA a piece for a sum total of 60mA per physical LED. If operating at 12VDC, 60mA equates to (12V * 0.06A) = 720mW per LED.
The strips vary greatly between manufacturers in terms of the LED density (number of LEDs per meter of strip) but 30 LEDs/m is common so I will assume that moving forward. If you go with a different density, you will need to re-run my calculations. 30m of strip multiplied by the assumed density of 30/m gives a total of 30m * 30/m = 900 LEDs. 900 LEDs * 0.72W = 648W which equates to 54A @ 12V. A power supply that can handle that load is not hard to find (PC supply would work), however, dimming will become a bit problem. The individual circuits being dimmed contain only one of the dies from each physical LED and therefore only consume 1/3 the overall current. In other words, the above example would require three 20A dimmers, one each for red, green, and blue. Here is an example of a dimmer that would handle this type of application.
A typical rheostat-style round-knob wall dimmer commonly found in homes will not be able to handle multi-channel dimming to provide you with color mixing for the RGB output nor will they operate at 12VDC. There are a few special models out there that will support 12VDC but the 20A demanded by your application is above what they can handle and you would still need 3 separate dimmers.
A much simpler method would be to go the route of a single color such as warm white. Because there is only 1 LED die per physical LED they only consume 20mA per physical LED. Assuming the same density as above this would equate to a total current consumption of 18A. This would allow you to use a smaller, cheaper 12VDC power supply. Here is a dimmer that would be used to control a single channel.
Long story short, if you're looking for a simple and inexpensive solution, your best bet is to go with a PC power supply rated at a wattage above what your overall strip will require. I have used these power supplies in projects such as this with great success. The yellow wires provide 12V and shorting the green wire to ground (black) turns the unit on. Note that these supplies typically use 16 awg wire which is only good for a sustained load of ~10A. Simply cut the connectors off and use 4-5 yellow/black wires in parallel to share the load.
You may also need to verify that 30m of LED strips can be connected in series. This is information that the manufacturer should be able to provide to you and is dependent upon the trace sizes on the flexible strip and their corresponding current carrying capacity. You may only be able to run a maximum 10-15m of strip. In that case you could either place the power supply in the middle and run 15m either direction or run a wire from your power supply to the second half of your run.
............................Strip.1==| Power |==Strip.2............................
| Supply |
| Power |==Strip.1............................ Strip.2............................
| Supply |======================================='
Disclaimer: The numbers in the calculations above are based upon common assumptions. Be sure to check the details from the manufacturer of the LED strip you intend to purchase and run the calculations yourself for total current consumption and voltage requirements. The manufacturers are eager to share this information either on the product page or in the accompanying datasheets. Most manufacturers values should be close to my assumptions but there is no definitive standard under which these LED strips are produced.