A needlepoint stretcher rather sounds like something you would cart the sick and injured away on doesn't it?
It is really just four interlocking pieces of wood that form a frame. The canvas is tacked or stapled to the frame and held taut.
To ensure the canvas is stretched symmetrically, mark the center of each side of the frame and of the canvas. Line up these marks when you tack the canvas to the frame.
Tip. If you put masking tape over the tacks or staples then you won't catch your thread on them.
Where Can I Buy Stretcher Bars?
Online sources are mscanvashouse.com, needleworkcorner.safeshopper.com and a-zneedlepoint.com.