The best materials for bonsai are collected trees and professionally grown pre-bonsai---both of which can be expensive. If you have guidance from a grower in your area that can help you through the process from beginning to end this would be a good way to go. However; if you are on your own and trying to learn by yourself you have to understand that it is very likely that you are going to kill your first trees.
If these first trees are $200 pre-bonsai or $1000 yamadori (collected trees) the cost of learning all of a sudden becomes marginally high, depending on your descrectionary funds. Odds are as of now, considering your experience, you would not know a good pre-bonsai from a ham sandwich. If you order your pre-bonsai from on line, unless a photo of a specific tree, ( and not a photo representation of stock in a particular category) is provided, you are still buying a pig in a poke. I realize there are some very reputable vendors out there, some of them have been suggested, and buying from them will probably not get you cheated.
Once again you must remember the attrition rate can be high in the beginning. I have always been a proponent of the nursery trade. There are a lot of things wrong about nursery trees that make them difficult as the years progress but the cost of losing a couple of them is small. In short; get your hands dirty and your mind around the process before you start killing good trees and losing a lot of money. It's easy to sit back and wish you had started with better trees instead of wasting time with trees that were going no where. Sad thing is it is usually those who have gained a good deal of experience that are able to make that assessment after they have destroyed a couple of trees, or worked on useless trees that they now dispise. The fact is this: Killing a few trees and learning about frustration, disappointment and failure are the dues that you will have to pay to get into this hobby.
Almost anyone can fly an airplane once its in the air with a few easy lessons. Learning how to land and take off makes getting there far more difficult.