I bought my waterbrushes at AC Moore, I think they are NIGI. They are fine, but I am sure there are better out there. I have both a medium and a fine brush. Sometimes I wish I had a wide one for doing backgrounds.
I do store the water in the brush, but sometimes there is too much water coming out and I guess I am just used to drying off the brush part on a paper towel to suit what I want. Some people (Jennifer McGuire for one) say they do not even put any water in it, but dip in in water like a regular brush. That seems to defeat the purpose for me.
On my brush I do have to squeeze very lightly to get the water onto the tip.