if it’s not windy where you are a heavy clear or semi-clear plastic would be more workable and cheaper. Home Depot or Lowe’s have plans in the garden department for as easy or extensive a greenhouse as you are looking for

My experience is in building materials rather than plants. One low cost option is a corrugated polycarbonate. This is not as thick as you may want, but it does have its advantages. While the corrugated stuff will not have the impact resistance (hail or falling tree limbs) like a thicker flat sheet, it will span greater distances than the flat material without sagging. A sample can be found at
I’m sure you can find it at other stores as well.

If you are looking to spend more to build a very high quality green house I would suggest that you do a little research on the types of polycarbonate manufactured. There are many types of polycarbonate sheets made that you will not encounter at your typical hardware store. Some are specifically made for greenhouses.

As for purchasing, that’s up to you. More than likely, the best place to buy will be a local hardware store. Call a few (small and large) to get prices to be sure you are a good price. Since sheet polycarbonate is so heavy and bulky, freight would probably be to much if you ordered online.

Charley’s Greenhouse and Garden …. charlyesgreenhouse.com.
I built a greenhouse in my backyard last summer and I found this site has great tips along with many useful products for the greenhouse and backyard, you can also request a free catalog.

I apologize, but I don’t know of any cheap, but good quality, polycarbonate glass.

As for free plans for a greenhouse, try here: