what are you making? It really doesn't have to be exact...just water your glue down until it's easy to use with a paint brush...like the consistency of syrup I guess? Even if it's thinner it's fine
You can find it almost anywhere they sell laundry detergent, that's where I always find it. I love making goop, so much fun for the kids and adults. LOL.
Re: Where to find liquid starch?
you should be able to find it at target/walmart someplace like that.
You can also use elmer's glue mixed with water (mix to the consistency of liquid starch)
I thought it was just watered down glue, but I've never actually seen/used liquid starch before, so I have no idea what its consistency is!