Mottling without an airbrush?

It is possible and relative easy technique to do. I use a brush with short stiff bristles. I dab it in the color I am applying then remove most on a piece of paper laying on the bench. Then apply the remaining to the model by dabbing the almost dry brush to the model. You keep dabbing until you get the coverage you want. Reload the brush as needed. Its a similar technique that is used to do walls in a home.

Practice on a test mule.

Rotating the brush in your fingers will give different patterns as you dab....dab lightly!

You can make a short bristled brush by giving an existing brush a haircut. Trim the bristles, experiment...still not working well, trim again.

One technique that helps is to minimize the paint on the brush.  Do this by shaking the bottle hard, then dipping the brush in the cap. You have to keep putting cap back on and shaking, because you use up the paint in the cap fast.  And as the previous post says, use a shorter bristol brush.

Before I had an AB I used Gerald's method. Its  long and tedious but it works. Just as if you were dry brushing.