Density of non-ideal vapor mixtures

I'm looking for a reasonably accurate density of a mixture of two gases that act quite unlike ideal gases (generally steam and water-miscible solvents). We can get a first order estimate by taking a molar average but that can be an order of magnitude off, as in the case of water/ammonia. 


Literature is the obvious place to start but I wasn't able to find any general sources and only a very few individual pairs in a narrow temperature/pressure range.


I would guess you should be able to derive an excess volume from fugacity but I haven't been able to find a formula that doesn't take even more difficult-to-measure inputs.


Standard modeling references like Refprop seem to use an ideal mixing model which isn't right.


I can also imagine measuring them myself or doing molecular dynamics modeling but these are both quite a bit more work than I was hoping for.


Are there any literature sources for this data that I couldn't find? Anyone have an idea of another way to calculate it?


