I've got a polytunnel & it is just fab.The best thing I ever bought.Mine is big 20ft x 40ft & bought it from a nursery which was closing down,for £50.The cover came from Northern tunnels and I've also used a company called Citadel too,who were very good.
It was quite easy to put up,as we marked the pieces as we dismantled them,so we knew what went where.I was lucky getting hold of it,my dad & I had been watching the newspaper ads for months,but any for sale were snapped up very fast.Then I heard that a nursery down the road was selling up & went to see the owner for a chat-result!
At the minute,I'm growing tomatoes,chillies,strawberries,tomatillos,peppers,courgettes,squashes,cucumbers,pumpkins,beans,potat oes & sweet corn.It has been a bit of a rushed job this year,as we only got the cover on in May,rather late.Next year I plant to build proper beds & get a fig & nectaerine in there too.
It is great-I had crops way before I normally would,apparently Dec & Jan are the only 2 months that you can't grow much,beause of the light levels,but you can extend the growing season no end.Yesterday I planted salad,carrots,beans & wallflowers.I sell a lot of my produce too,as there is more than enough to spare.That helps pay for next years seed.
A good book to give you a better idea,is 'Growing under plastic',by Bernard Salt.He covers polytunnels & greenhouse,with what you can grow when.As for finding one,perhaps 'Farmer & Grower' or one of the horticultural magazines might be worth a try,or asking at your local garden centre if they will be getting rid of any in the near future.