How do I drive 12 strings of 12 leds each?
Where do I start for something with a low cost?


You start by understanding what you need. If the forward voltage of each LED is 1.8V and there are 12 in series in each string, you need a supply voltage that is at least 12 x 1.8V = 21.6V. 

But you will need a little bit of overhead to control the current through the string of 12 so I'd go for a 24V DC supply and a resistor in series with the LEDs. The resistor will develop 2.4V across it (24 - 21.6) and have 20mA flowing through it. The value of resistance that meets those demands is 120 ohms.

The power taken by one string is 24V x 0.02A = 0.48W.

The power for all twelve strings is therefore 5.76W. 

The current is 12 x 0.02A = 240mA so you should possibly look for a wall-wart that produces 24V and has a power rating in excess of 6W.

In case your LED voltage might be higher (such as 2.1V as per the details in your question) then the max voltage will be 25.2 and you'd realistically be looking at a 30V dc supply.

If you think you need a higher current than the 20mA then use the method shown in my answer for recalculating \$ R_{series} \$