It's around 25 years possibly longer, since I last bought colour couplers so I've no idea now what I paid for them. There are/were alternative couplers particularly for yellow, somewhere (in storage) I have my notes from when I was doing the research into toners with the alternatives listed. It's worth checking the colour couplers used in Kodachrome processing as these would have been made in larger quantities, prices for older couplers may be high as there's almost no demand/production.

You'd need to cross-check the MSDS or other data-sheets to check the alternative names for these compounds.

Stability of any colour coupled dye is partially dependent on storage and exposure to light but a dye-coupler toned print should last as well as a colour print under similar conditions.

Ian