I buy most of mine from MessySupplies- it's pretty much 'add water and mix' www.messysupplies.com You can also get branded products such as Slube or Slime Baff.
Making it from scratch- food thickeners such as xanthan gum or guar gum are readily available. Powdered clay such as bentonite can also be used. If you check the DIY section of MessySupplies you'll find the chemical names of the active ingredients for the different gunges/slimes available, which you can pick up from a chemical supplier (or eBay) - https://www.messysupplies.com/collections/diy Note: flour (such as in cake batter) can 'ball up' and stick to hair when your trying to wash off, making cleaning exceedingly difficult.
For colouring use tempura paint (such as children's poster paint) as this is both non-toxic and mostly non-staining. Food colouring is non-toxic but more likely to stain. Avoid household/industrial/metal paints as these may contain toxic chemicals.
For effects; porridge oats/rolled oats are great for adding a lumpy texture. A small quantity of borax solution turns guar gum super stringy/stretchy.
Mixing: check individual instructions- sometimes you can add water and hand mix, for most you'll need to sieve the powder into water and/or use an electric mixer to avoid it clogging into lumps.
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