Where can I find cake dowels that are longer than 12"?

If you just want wooden ones you can go to the lumber supply like Lowe's or a craft store like Hobby Lobby. I get my big center dowels at Hobby lobby.

I was told not to use wood from hardware store because he wood could have been chemically treated.

What about the ones at a craft store? I hadn't heard about that - though you could probably wrap them. I use bubble tea straws for my main dowels, but I use a large wooden one for my center.