10-bit DAC suggestions

I need help. I am running a threshold test using psychometric toolbox.
The problem is that my video card is 8 bit and thus I dont see all the
shades of gray. I heard that I should upgrade to a 10-bit DAC. I am new
in this bussines. Does any one know where I can find a 10-bit DAC? Is
there any good video card I can buy that has a 10bit_DAC? Of course
price is a concern too.
Sincerely,
ELios