Yesterday I bought some steel wool and copper BBs. A guy from a scrap metal business not far from where I work replied to my query that he has aluminum and copper shavings he'll sell to me, so I'll pick that up tomorrow.
Tomorrow night I'll make my first batch of TB's. I'll try all 4 materials and try and see which one works the best. I also saw that you can get aluminum powder to put in your resin mix. At some point I may try that as well.