It really depends on what state you live in and what part of the country. Each state has their own regulations and such for chemicals like these. I remember when Windy first mentioned that he couldn't get 3608S on the east coast anywhere and it was replaced with 3608W which did not work at all. Lots of people, including me, have had good luck with Kleen Strip lacquer thinner from Lowes or Home Depot. Even that may vary from state to state on what's in it. If you look at the ingredients on cans of most hardware store lacquer thinner, it will just say "100% Acetone". A few years ago I went around to different store in my neighborhood and bought quart cans of different brands of lacquer thinner and that is what they all said on it. Kleen Strip has some other additives. I found someone selling off gallon cans of an off brand lacquer thing on FB Market place that just said "Brush Cleaner" on the label that made heavy use of the Spanish language, and in the ingredients it read just like Kleen Strip. They may have changed the name to brush cleaner to get around a rule or regulation. I tested it in some clean SIG dope and it thinned out nice, and I'm letting it sit to see how it ages in a jar I'm testing with some cardboard seals to see if it stays the same or dries out. In this day and age, I would just say good thinner is where you find it, and you just need to do some testing to be sure.
  Type at you later,
   Dan McEntee 

Back when I was painting on a more regular basis, I was using Sig dope and I was able to use DuPont 3608s as a thinner as an alternative to Sig thinner. Is there anything like 3608S available again?!!

It's still available. It is branded Cromax instead of DuPont. Go to the car paint store.