Most pipe is sized by ID (inside diameter).  It will be difficult to find exactly 6 inch OD.  You could try some schedule 80 or higher 5 inch and that should be almost 6 inch OD, at least within a quarter inch or so undersize.  Your best bet would be contact your local heat/ac contractor and find some PVC Ducting.  You can get that in many large sizes and it is measured in OD if I remember correctly.