Stainless steel leader wire. The light stuff, I can't think right now the diameter, but it is rated at about 80lb test or so. I use round-jaw pliers for my loops and other small fine-nose pliers for other forming tasks. No need to buy a wire bender. A 25 yard coil of leader wire costs about $4 or so, and will make hundreds of spinners, so cost compared to mono is not an issue. The other problem with mono is that using a correct clevis for the spinner blade will eventually wear through and weaken/break the mono. There are plastic quick disconnect clevises which the walleye guys use for crawler harnesses, but they are not well suited to small trout spinners.