There are several different approaches to glazing a greenhouse, and
most books on greenhouse design dedicate at least a chapter to this
subject. The usual options are
glass: nice but rarely used anymore
poly sheets: in 5 or 6 mil thickness (A mil is some silly unit dating back
cubits or bushels or similar nonsense), cheap but it tends to fall apart
in a year or so.
modern plastics (lexan etc): the best, but pricey
fiberglass: Economical, light, easy to work with.
Fiberglass can be gotten from greenhouse supply stores or some large
hardware stores. In this part of the U.S. and in California are huge
hardware/building supply stores called Home Depot or Payless. Try
calling a few building supply places. The individual fiberglass panels
are available in 4' wide strips, in lengths of 6, 8, 12'. These
sheets are either corregated or you can buy big rolls of flat material.
It cuts well with a hand or power saw. Make sure you buy the transparent
kind meant for greenhouse use. It is UV protected so it won't get opaque
with age. It is much more expensive than poly, but hail and storm and
sunlight resistent. Once in place, it requires no care and lasts for
at least a decade.
Regarding vents, unless you live in a very brisk place, I think that
some sort of cooling is very important. At least a vent. Greenhouses
heat so quickly from sunlight, well, there ought to be an effect named
after it! :)
Barry