I believe there are some that use thimerosal instead

I have seen thimerosal also like Tomás Allende and I have also called the vendor and asked for the sodium azide-free version which was not in the online catalog, but that was from a purified IgG made from pooled human sera.

Many companies can give you a azide-free or other 'custom' formulation, just give them a call. They may charge a fee for this, so dialysis may be your most economical option.