Rather than attempting to adjust the pH, why not replace the container mix in both cases with one designed for acid loving plants? pH adjustment is never all that easy and is more prone to error in a container than in open ground. A mix designed for ericaceous plants is also likely to provide the excellent drainage and aereation azaleas need.

Ironite is a fertilizer plus iron sulphate, I think, and does not by itself acidify. It will green up chlorotic leaves if, and it's a big if, the chlorosis is due to soil that is too alkaline. Its use would be unnecessary if you use a potting mix as suggested above. Elemental, aka garden, sulpher is probably the best way of lowering pH, but I wouldn't use it in a small container.

You could also get those azalea fertilizer packs that are intended to feed as well as acidify the pot environment (or ground around the azalea).

Mulching with pine needles might also do the trick, but that is more of a long-term option as the needles break down over time.