There are alot of tunnel companies out there. I love Rocket Tunnels. They have a lightweight one and a more heavy duty one.
I have one heavy duty one that has lived outside in the elements for six years (this is california so not terribly harsh) and is still in good condition. I have also gotten one of their lightweight ones (yellow) and it is holding up really well. What I like about the lightweight ones is I can carry it so easily. I can store it on a self in my shed because it is so light.
A couple years ago I needed another tunnel because Tazz was having trouble with dark tunnels. I got the heavy weight dark blue. I also splurged and got some tunnel bags as well. I LOVE my tunnel bags. They were expensive, but boy are they easy. I filled up trash bags with sand and put that in there and they are holding up great.
Don Soucy, the owner is a great guy and also runs dogs in agility. If you have questions, call them. And for those of you lucky enough to live on the east coast, he goes to quite a few shows over there and you can save on shipping.
You can set up your tunnel under the tree and instead of a train, your dogs can go round and round!