what i do is go to a tractor dealer/service department and ask for them. 55gal metal are used to haul oils, 55gal plastic are used to haul cleaners. so you got to clean them out, but they are happy to get rid of them.
I went through the same issue, and finally found one at a local construction company. They service their own trucks (large concrete co) and get windshield washer fluid in them.
i got mine from the automatic Car Wash place. It will either have contained soap or wax. Try to get one that contained soap. add 2 gallons of water, swish it around, then dump into a 5 gallon drywall bucket, place lid.
Now you'll have some car soap for rinsing off your equipment for a while.
They use a high concetrate, so even adding the water, you still will have to dilute further when you use it, or your neighbors will refer to you as Mr. Bubble. Ask me how I know?