DTT and IAA have different functions. DTT is a reducing agent to reduce disulfide bonds and protect Cys residues from oxidation. IAA reacts covalently with Cys residues, blocking them.

Joko Priyanto Wibowo already suggested alternatives to DTT as a reducing agent. BME (2-mercaptoethanol) is much cheaper than DTT, but it smells much worse because it is more volatile. DTE (dithioerythritol) is basically the same as DTT (dithiothreitol), just a different sugar. Reduced glutathione is another possibility.

An alternative sulfhydrl blocking agent to IAA (iodoacetamide) is N-ethylmaleimide.