Attaching a flue to a solid chimney liner?

We are installing a log burner and now have everything prepared to put the fire in, we have the log burner and have a 5" flue pipe but need something to increase the width of the flue pipe as the solid chimney liner is 8". We have seen a stainless steel fitting that would suit the job but it seems very expensive at �60 for what it does. Has anybody had to get anything like this before? your help would be appreciated with this.