Gary,
I make my own washes - I shave/scrape soft pastels, mix with water & a drop or two of washing up liquid ( removes surface tension ) & use the same way as a commercial product.
I bought my pastels from Amazon, but ebay, and local artists/office also stock the sort of thing...................
As long as you have the dark colours + white, you can make all washes - they last a long time - some of the colours will never be used, they cost about the same as one bottle of premixed stuff.
It takes a bit of experimenting to get the right consistency, they're not as durable as normal washes, but, I always seal in the washes with varnish anyway, so that's not a problem.
This the home made wash on the just finished Tie Fighter
Dave