Disclaimer - these are products I buy myself, I have not been given any form of reimbursement for writing about these products. Also, I definitely haven't tried everything that's out there! This is just what I use. :-)
In My Stash
Pentel Aquash Water Brushes
When I first started watercolouring stamped images, I used the Aqua Painters from Stampin' Up!, which are made by Pentel. They're a nylon brush with a 10ml water reservoir, which means you don't need a container of water nearby to watercolour your images. I always favoured the smaller nib as I found I could control it a lot better for smaller areas. So I was pretty excited to discover that Pentel actually make an even smaller nibbed water painter! Straight from Pentel, they're called Aquash Water Brushes, and you can buy them individually. I bought mine on ebay HERE, although I've seen them in art stores since.
Although I have all 3 sizes, I predominantly use the smallest nib (fine), purely for more control. For large areas though, the bigger nibs are brilliant. I always have a few sheets of paper towel nearby so I can quickly clean and dry the nib if I want to change colours or the flow of water, which is often!