The image below shows two lots of wire, before and after straightening. 1 and 2 are thin brass wire. This straightened without any problem. One end in a bench vice, and a firm pull on the other end with a large pair of pliers.
3 and 4 are much thicker wire, but copper. This also straightened fairly easily, although the pull had to be firmer, but was OK even on this short length. With thicker wire, I have found that the longer the length, the easier it is to stretch.
I am not sure it would be all that easy to stretch thick brass wire, but as straight brass rod can be purchased cheaply enough, why would you want to stretch it yourself? But, as you have some, just try it, and see what happens!
As far as I know, you cannot purchase straight copper rod, but it is easy enough to make yourself.