Maybe here : http://www.bhphotovideo.com/c/product/882991-REG/Beyerdynamic_dt_770_se32_DT_770_PRO.html
To be clearer in my explanation :
The current I defines how loud your headphones will be. Your system delivers a constant voltage V.
With the Ohm-law, you can write I = V / R.
So if V is constant and R increase, I will decrease. So high impedance headphone will need more current to sound loud.
You also have to look at the efficiency of the headphones also known as Sound Pressure Level (SPL) written in dB for a certain amount of watts (typically it's written for 1mW). If your headphones is efficient (high SPL), you will need low current to sound loud.
EDIT : The 880 is surely the best of the lineup. But it's semi-open and leaks a lot. With a relatively low price and open, the best you can find on the market in my opinion is the HE-400, but since you're going <200$, it's a bit too pricy.
Edited by fishbone - 11/26/12 at 10:35am