If you already have a custom loop setup just finding the right GPU block for your card would likely be your best bet...
take a look at the sticky on the top of the forum for specific vendors.
If you do not, then you will need to look into setting up a custom loop and cooling your graphics card, and whatever else you decide you want to liquid cool (CPU, MB, etc.)
do you have a budget that you are willing to work with?
If you don't have a loop already set up then it will not likely be very cost effective unless you are interested in watercooling in general or want to set up a loop for more than just the graphics card.