Water Heater Duct

The flue pipe for our new water heater failed the township inspection. According to the local plumbing code, the diameter of the flue pipe should be 4". To fix the problem, we need to buy two 4" elbows, one 12" long 4" pipe, one 6" to 5" reducer plus 4" wye tee, one 30" long 6" pipe and one 6" elbow. Audubon Plumbing Supply sells these pipes and fittings. However, since this is the first time I work on stuff like this, I will buy from local Lowes or Home Depot because of their return policy.