Dog Runs
Ask my cousin about the carpet mills and he said that's what made him want to build his own.
1. Didn't move as easily, really needed an experience dog on it to keep it going.
2. The carpet wore out to quickly.
He also said that he generally sells them for $650, but for a bigger down say 80lbs that would be about $750. We're in CA, so shipping is approx $200. I know, ouch! But these things are light weight and very easy for the dogs to run on. The link below is a picture of one with soft wheels to keep the noise down, but without the panels to run on. I'll be getting pictures of a completed one soon (and video of dogs actually running on them). He uses lightweight Oak panels for the dogs to run on, very durable and they last a long time. I believe he also sends out spare panels that can be swapped out in the event one breaks. He machines everything himself, from scratch.
This treadmill is "in progress".
http://i78.photobucket.com/albums/j109/fuzytech/krossfire/pictures/treadmill-01.jpg
Disclaimer: I know some people get upset when someone tries to solicite work on forums so let me say this. I'm not trying to sell anything on here, nor am I trying to advocate for another seller. I'm merely giving advice. My cousin doesn't do this for a living, he actually works for a school district as a welder, locksmith, electrician, A/C guy. He's just your typical handyman. Once my pup gets old enough I'm going to have him build me one on a "family discount"

The summers here reach 125 degrees and 109 at night, so it's not always good weather to take your dog out for a walk. His dogs absolutely love them. In fact he has to hide them (the treadmills) in the garage because the dogs just go crazy when they seem them and don't want to get off.
BTW, if you do a search for Treadmills and Pits you'll find plenty out there to choose from.