Or if you just fancy ordering one in the UK First Tunnels have a great range.
There are hundreds of different garden greenhouses and polytunnels on the market, and which ever one you choose it will never be big enough, so make sure you build or buy the biggest one that you can afford, in the space available from the start. Mine is roughly 5m x 5m. 2.3m high in the middle, and 1.7m high on the sides. These dimensions were slightly governed by the wood that I could purchase from a large DIY store.The wood that the roof sits on were poles puchased cut in half, so there are no sharp edges.I secured the roof support poles to the center uprights with noggins, so there again would be a smooth run over the ridge for the plastic. For the sides you can use 50 x 50cm treated wooden lengths.
The reason I chose to build my own was that it is far stronger than most polytunnels, and to have one the same size would have cost nearly double. Mine cost me approximatly £500 in total.The other reason to have a wooden greenhouse, is that it is far eaiser to run wires down it for crop supports. 
The special plastic I used should last three to five years and is availible on line, just make sure you order enough to cover the whole structure side to side in one length. The ends you can fill in after.
As you can see the structure is fairly stong. I added so extra support trusses inside, to the roofing struts. I will have to change the plastic in a few years, so there should be no problem getting on the roof to do this.
It has now been up for a year, and we have had a bad winter with three lots of heavy snow, and very strong winds, so I feel it is a pretty strong structure.
