Just as with any other resource, find a node (anything above q10) and chase it down... go a few squares in one direction, test again, if its up you are going towards the centre, if down, away. Bear in mind that centre of node may be under ground where there is no option to dig a well.
Note that it's quicker to just travel along the rivers and use your flask to test water quality every once in a while. You can then look for nearby well spots that are potentially closer to the node center. So you can avoid building a million wells, unless you're trying to find HQ water in the middle of nowhere (read: no rivers and lakes nearby). Granted, your chances to find undiscovered good water in the "middle of nowhere" aren't too bad, simply because most people can't be bothered to build wells everywhere.
Cave water can be high quality as well, if you have access to cave networks and are willing to dig to a few to test them out. If you can hit a pocket cave with no entrances or exits that has high quality water or clay, you've struck gold essentially. The easiest quality node to not only conceal but protect as well. The problem being the absolute rarity of such nodes.