Glastar make a high quality lead stretching vice. A little more expensive than some but this one gets a really good grip and is made to last.
The vice has a hook to go over the end of your bench and a hole so you can screw it down. You push one end of the lead came into the vice and the spring clamps the teeth down to hold it while you pull the other end with a pair of pliers. The lead needs to be stretched to straighten it but it also stiffens it. Narrow cames need a fairly gentle pull but the wider ones take a good effort to get them to stretch!