It all depends what pipe you want. Big box is usually the cheapest, but they only have the "cheap" pipe.
IE pelletvent (big box) vs pelletvent pro
I've heard the cheap stuff leaks, requires a ton of sealing and is a pain, but for a garage the savings might be worth it to you.
If you want the good stuff the dealers use to install check out the site above or
http://www.woodstovepro.com/
Sean is very helpful and has helped many around here. Usually there is a discount code available so the price gets even better once you have him quote it. He is out of Ohio and thus shipping to me was very quick.
If you end up going with 3" pelletvent from the big box----I have a 3ft piece and a 45 that I would give you a good deal on. Came with the used stove I purchased.