I am going to pm you. This is an old thread. The trees I had for sale at that time are long gone but I do have a few now.
12”-18” in what dimensions? If it is height then it is very hard to say. This can be a 6 months old seedling that can be had for $5 or a developed tree for much much more.
If it is trunk diameter at the base, then this is a massive tree that requires special shipping arrangement.
Since you are so new here I can’t guess what you want. Sometimes it is easier if you tell us you target bonsai tree size and your budget. Then I can tell you what I have.