What do i need to know about shade cloth. New greenhouse is built and i'm learning

Its finished except for getting the electricity hooked up out there. I've got a temp controller which will turn on the fan. I realized real quick how important venting is. Its March in Arkansas and inside greenhouse will get to 95 degrees without a fan, which so far I have blowing in the open door. I'll be mounting in front of my upper vent Sat. Have a lower vent on N side and upper on South. Planning on putting in more upper vents. I was at a commercial greenhouse today and it was so cool inside without fans and only the front and real doors open but it had shade cloth.
So I'm thinking what to use. Hopefully there is something I don't have to buy since this greenhouse project has taken more money than i planned and a lot more time. So what can i use for shade cloth or if i have to buy it what do i need to know.

You can get mesh landscaper tarps cheaper than shadecloth sometimes--check a local farmstore or landscaper supply first so you know a baseline price.

How big of a house? Is it hoop or peaked? Is it film or solid panels? What do you intend to grow?

Many end up with 2 shadecloths--one as a light shade and one as heavier shade for the really hot days. We used to use cheap tarps (the $1 5x7's from harbor freight or remnants from anywhere) as the top shade. We put them up checkerboard style so plants would get mixed sun and shade through the day.

R. Scott has given good information. In Arkansas, you can also add a swamp cooler to one end of a green house and pull the air through with a fan at the other end. Most of the commercial houses I've been in here in Arkansas are glass with automatic vents every twenty feet as well as a swamp cooler setup and shade cloth. Mostly it will depend on what you are growing to determine how much ventilation, shade, cooling that you will need. I have plans to build what I consider a small hoop house (6' wide by 30' long using 20' 1/2 inch pvc for the hoops. Both ends will have doors and vent windows with fans, as well as a shade screen that can be moved across for shade as needed. http://www.shadeclothstore.com/depts/wovenshadecloth.html is one place you can order from with not bad prices, in a green house these materials will last years when cared for.

I put up some shade cloth which helps but it was still getting too hot in spring inside. Put more vents in, running fans. I got it so it was only 10 degrees warmer inside. I looked into swamp coolers a bit. Never found out how much humidity in the air is too much. I noticed humidity in early spring wasn't too much here. Might rig one up next fall. I didn't have my greenhouse set up correctly and water accumulated and pulled the plastic down, letting more water fill up till my 1in pvc was bending and breaking down the the ground. Disaster. I did learn that if i have it wide like i do (18ft) i need to support in the center. I also am going to put in some more supports as my pvc was a bit over 2 ft apart. So add two more pvc sections in the middle, support in center and make sure its tight and one of our heavy rains won't cause another disaster.
I've got the top off now for the summer. If i had it to do over I'd use 1 1/4 or 1 1/2 pvc and do a better job with the framework. ITs been a big learning experience and so far money pit.

+1 on the greenhouse being a money pit. also I noticed I said 1/2 in. pvc when I should have said conduit. The metal will do better, longer in that smaller size that I intend to use for our starter house. Since you already have the fans, all you need is a frame, a small 12 v water pump (like for a boat bilge) some tubing and an excelsior mat or pieces of indoor outdoor carpet cut to fit your frame. I used to have one and found that the in/out carpet doesn't raise the humidity as much as the excelsior mats do.