How big an aviary are you looking for and at what cost?
If you just mean a flight cage, then a single or preferably double standard flight cage (the ones made by HQ and AE) would be fine for small parrots. HQ also makes a larger sized, more decorative flight cage also with safe spacing.
If you mean an actual walk-in aviary, Centurion cages makes some in 1/2" spacing... not exactly cheap at $1200 for the smallest 5x5' cage.