I've helped build tonnes of small Aviaries/large cages, it is not hard at all. I don't know where you live though, so i can't help you find materials.
Since the minimum cage sizes for Macaw-sized birds is about 6.5 square metres over here by law, i suppose we're more used to not buying pre-manufactured cages. My pois' cage is 2*1*2m and made out of pre-made sections that are very easily put togheter, took me half an hour alone to do it.
What type of Aviary is it you're building? Outside or inside, how large, and where will it be located?
Is it for macaws or smaller birds?
The best materials and construction methods vary, so that would be good to know.
A friends cage, for the front he used a birdproof paint over a galvanized net. Housed an Umbrella Cockatoo and a Military Macaw.