Something's weird there. I bought chrome alum without any problems from my usual chemistry supplier. It wasn't especially expensive. It's not toxic, or regulated in any way AFAIK. It's not labelled even irritating here, but I can see that some MSDS's state that.

140 USD for 250g sounds quite high. I paid 39,80 EUR for 500 g and I've understood that generally you have lower prices there. I would have bought less but 500g was the smallest bottle.

It can, like other hardeners, be added to the melted emulsion itself just before coating. Just make sure to mix it thoroughly. I've not tried it yet, I've been using glyoxal as a hardener.