I love gunge powder, but getting shipping from England is a pain.

I started searching eBay for hydroxyethyl cellulose powder with a "ships from US" filter and bought some. The confirmation email from the seller had a company name/website link: https://www.chemtechproducts.com/

My first bucket didn't go so well, but I think I messed up the timing on the mixing and let it gel in the bottom. 2nd one went much better with the same stuff. It started to get a little granular at one point but straightened itself out.

I've been coloring it with craft paint that came in a big jug with a pump dispenser and a "4+" age limit on it - the only warning label is the one that says there would be other warning labels if there was anything to warn about, but there aren't. Remember to do allergy/stain tests!

Now, boring economics:
Shipping 1Kg of HEC from England totals about $67 US, or about $10 for a 10L bucket of 15g/l gunge.
5lbs (~2.3Kg) of HEC from this chemical supplier is $81, or ~$5.25 for the same bucket.
Now, to be fair, you do approach that price point if you get 5Kg shipped from England to the states, but that's a lot for the home enthusiast.

Happy holidays, stay messy.