This is one of those instances where, going into a thread, I think I'm in for one thing based on the title, and then it turns out there was a very crucial element that I had not foreseen...
Seen this question a few times on the forum, they'll be a few more answers buried in here.
The recipe I tried used MS gunge (white, thick), and MS Vivi extreme (uncoloured). Mix the MS gunge extra thick, using about 6 liters of warm water. Mix the first part of the Vivi slime in a different bucket, about 8 liters of water. When the MS gunge is fully thickened add it to the Vivi slim and blend together to break down the MS gunge into small lumps. Finally add the Vivi activator a little at a time to get the right level of stringiness- about 1/2 to 2/3 of activator.
You should check out my recent video and send me a PM.
There are a bunch of different ways it can be done, but I'd be willing to disclose some of my secrets.
In BULK, it can be made as cheap as $2-$3 per gallon, but if you are only looking for 1 gallon worth you can usually find the supplies on Amazon or somewhere else. Just takes some practice to mix it right.