What is green stuff?
Green stuff is a nickname for a product known as Kneadatite Blue/Yellow Epoxy Putty. It's originally meant for basic home repairs, such as sealing cracked pipes.

But hobbyists like it because it can be sculpted into all sorts of shapes, it holds details really well, it has a smooth texture, and it gives you a fair amount of time to work with it before it starts to set.

It comes in two parts, either as separate sticks or as a long roll of "tape" with the parts next to each other. You mix the blue part together with the yellow to activate it and you get green, hence the nickname.

Where can I buy it?
Kneadatite is supposedly available at home improvement stores, craft supply stores, or local tabletop gaming stores. I say supposedly because I didn't have much luck finding it in my area, so I ended up resorting to Amazon.

Other advice!
Keep a bowl of water at your workstation when you use green stuff, and if you have sensitive skin, wear gloves. Green stuff is sticky, so be sure to keep your hands and whatever tools or surface you're working on moist.

Lubricants like petroleum jelly will also work, but could leave residue on your material.

If you're buying green stuff as a side-by-side ribbon, be sure to cut out the bit where the blue and yellow parts meet before you start working. It'll have begun the curing process and won't be at the same consistency as the rest of it.

Don't mix together too much green stuff all at once. You can always mix together more if you need it, but any leftovers will just go to waste. This also ensures that you keep working with a relatively fresh mixture and have as much time to work with it as possible.

On a related note, green stuff doesn't come with a pause button. Make sure you'll have plenty of time to work uninterrupted before you start a project, because once you mix the parts together, the clock starts ticking.

Sources!
Here's advice from the official Kneadatite website in .pdf format: www.polymericsystems.com...

so it's like Apoxie

Yeah, basically.

okay

so then you need to wear like gloves or something when mixing? or no

The website recommends it, but I don't use them and I'm fine.

But if you have sensitive skin, then yeah, I'd say do it.

Side benefit: not getting fingerprint marks on what you sculpt.

you can smooth those out with water tho

True! But you won't miss any and then discover them later after it's cured.

fine grit sandpaper could snooth them out after that tho

Also true.

So really, it all just boils down to personal preference.