Hi,
I have been interested in getting into bonsai for a few months. I have been lurking in these forums and trying to learn what I can. I have also signed my daughter (Age 15) and I up for a bonsai class at my local nursery. One thing I have learned is that I need to not only learn the techniques but also how to care for trees in pots. I have some basic vegetable gardening experience. Since there is going to be a learning curve I'd like to have a good supply of average but inexpensive material. The material doesn't need to be great just something to learn on. I live in the north east of the United States so it is a temperature climate with maple, oaks, and white pines on my properties along with a flowering tree I am still trying to identify.

My question is this. Is it worth Air layering some 25 to 35 mm specimens? Or does that process work better with thicker material?

Air layering with that size is fine, depending on the species though it can take anywhere from a couple months to a couple years. It's a great way to add a lot trees to your collection quickly and cheaply.
what species are you trying to air layer?

It is the white flowering tree. The leaves have are just starting to come out. I hope to get a better look this week to try and identify. I read I should start the air later after the leaves have set. Is that correct?