Root trainers are quite expensive, but you can easily re-use them lots of times if you are careful. In addition, they are made of recycled plastic. Many people say cardboard toilet roll is a good alternative. However, you do not know what chemicals were in the pulp or what glue has been used. In addition, you don't get the air pruning effect as the rolls and compost have to sit with their bottoms in a damp tray. Lastly, the cardboard is prone to going mouldy.
The right kind of quality sticks, compost and tools really are more exciting than new shoes.