have a look at messysupplies.com they have a few different types and even some videos showing the different appearances of each type gunge.
I usually go for the "Thick" Natrosol gunge - you can see the effects in some of the albums on my profile - more recent ones have been more like "standard" thickness, which can still be thick. Plus you can easily mix it by hand just be adding water to the power.
Methyl cellulose is a lot harder to mix, texture wise there is a difference but it's not huge. We've got methyl cellulose, carboxyl methyl cellulose and hydroxy ethyl cellulose (natrosol). There isn't a huge difference between any of them. We currently have Hydroxy ethyl cellulose on a special offer at the moment of just 7.99 for 24 litres of a very thick Gunge. It's EXACTLY the same as Natrosol but a different trade name and produced in the EU rather than America. Getting great feedback on it with people saying it mixes easier than natrosol and others getting 12 litres of a thick Gunge.