The last 3 foot (the part exposed out the roof) of my chimney pipe is rusted on the outside and needs replaced,I believe it’s triple wall chimney pipe, the inside diameter is 8”, then it’s like 11” and then the outside diameter is 13”, this is the exhaust on my fireplace insert, where do I find this pipe?ive checked everywhere in town and when I talk to sales reps online they give me the run around and I just need to fix this