Most agility equipment isn't stuff that can be left outside. Some pieces are very heavy, too. As Bruce and others note, basics are easy to build and if you want to buy, agility equipment is easily available all over the web in the UK, US etc.
One caution: Please do NOT do agility on a course unless you have done some classes with a proper trainer FIRST!!!! And do NOT allow dogs to do courses with people who have never done any agility, or if you do not know how to set up and train a dog to a course! Simply thinking it looks fun is not really acceptable. A dog could be very seriously injured doing agility improperly with lifelong damage to bones or joints -- if jumps are too high, if it lands the wrong way, if it doesn;t know how to *safely* stay on and get off items like the dog walk. Jumps must be appropriate to the size and weight of the dog and this is probably a lot lower than most people think is OK, if you haven't done courses with small dogs like cavaliers.
In short, just as you wouldn't head off to run a hurdle course or use gymnastics equipment without training on how to do it correctly and safely, nor should people's dogs start running around agility courses without any training or knowledge about agility safety and at what age it is appropriate -- and how much is too much.
Be aware that if you are making agility equipment and don't know much about agility or equipment requirements, it probably will not have the safety features that are built into good equipment -- displacement bars that fall in a collision, etc -- so you need to be careful about how you make things and what pieces you do yourself. Also, dogs are trained to observe contact points on equipment primarily for safety, not just because it is part of competition. If you don't understand what these things mean, take some agility courses so that you do before running your dogs or anyone else's on a course, professional or homemade.
I cannot stress this enough.
Doing small fun things like very low jumps (a few inches) and a tunnel are fine, but even then with jumps you must know some basics about safety and dogs under one should really not be doing jumps at all. And it actually takes a long time and many gradual steps to teach something like weave poles so expectations need to be in line with the dog's ability to learn.
In general trainers require dogs to have passed intermediate level obedience BEFORE being allowed to start agility -- and they must be at least 12 months old.
See:
http://www.isnare.com/?aid=179915&ca=Pets
http://www.dogpatch.org/agility/FAQS/faqintro.html
http://www.diynetwork.com/diy/lp_dogs/article/0,2041,DIY_14061_4202941,00.html
http://www.dogwise.com/ItemDetails.cfm?ID=DTA214