didnt sell pvc pipe in 6 inches outter diameter
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.