Shade cloth should be readily available from plant supplies or hardware stores
Polyfab USA has a range of colors and shade density -
http://polyfabusa.com/shade-products/monofilament-shadecloth/
They list 4 distributors in Texas -
http://polyfabusa.com/polyfab-shadecloth-distributors/polyfab-distributors/texas/
Those entries are just from an online search. The suppliers may not deal with smaller orders so check your local hardware and big box stores. You may have to look further as many only stock higher density shade cloth that may be too shady for good bonsai growth but as mentioned above you may be able to get around that by making the shaded area a little smaller so trees get more direct sun to compensate for the extra afternoon shade.
There are probably other manufacturers and suppliers.
Your structure can be as complex or as simple as you like. Large hoops are simple and easy to cover. Timber or steel pergola structure looks better and can be stronger if wind is an issue. My structure here is just light weight steel tubing on steel pipe uprights. The shade cloth slides over the top in summer and tied at the corners and at intervals around the edges. Easy to put up and easy to remove for extra winter sun.