Where do you buy slime or gunge from?

That entirely depends on what country you are in, provide some location info and people will be able to offer advice.

Splatter Emporium has great slime. When I want real slime, thats where I go. Otherwise I use cake batter.

X2

Speaking as someone in the US, Splatter Emporium is the only option I know of. Although, I think some in the UK can ship to the US. Kind of want to try to VEX gunge, but no US shipping...not really sure if there's that much of a difference, but it does look like their colors are more vibrant.

If you're based in the UK, MessySupplies is the premier supplier, they'll sell an assortment of slimes and gunges, as well as mixing tools, tarps/dust sheets, pools etc...
They offer international delivery too, but this might be quite expensive.

MuckyPup is a second UK supplier, haven't used them personally but have heard great things about them.

More generically, Amazon/eBay you can get Slime Baff/Geli Baff (kids product), Slube (Adult version of Slime Baff), as well as the raw uncoloured ingredients for gunge.
Supermarkets sometimes sell large packs of food thickener like xanthan gum, which makes some pretty great gunge.
Do NOT use regular (wheat) flour, as the gluten forms into literal glue and binds to everything. It will destroy your hair. I'm aware of a few unlucky ladies who had to shave themselves bald due to flour based slime bonding to their hair!!

You can also make gunge with PVA glue and borax, most craft shops can stock this as well as online retail.

Colouring your gunge is best done with tampura paint/ children's poster paint, because the pigments are non toxic and designed to be washable.
Food colouring works, but is much more likely to stain.
Household dyes and paints can contain toxic chemicals and probably will stain.

x3

Perfect gunge!

X2, where we get our messy stuff for lives, etc

We are likely to be relaunching the Gunge Shop in the next month or so. Prior to messy supplies we were the main and probably by far the cheapest in the UK.
Will probably be launching our Deluxe Xantham Gunge, if you ever tried our deluxe easy mix gunge this will have a similar feel and opaqueness but at a sensible price.
We will also be selling the powder paint colouring for anyone wanting that as well. I still have around 400kg of powder paint alone in stock.

I've used gunge from several suppliers and the easiest to mix currently is Messy Pup's.

Not sure if time has an effect on gunge, but stuff that i've had for a while tends to lose it's easy mix ability...

We've used messy supplies gunge for years, it's always been our go to.
We've tried messypup too and their gunge is excellent quality with very vivid colours.
We want to try vex but we've not got round to ordering it yet.

The main thing I'd say is find the one you like best, get some from each and just have fun trying them out.

What you really need is a good way of mixing, we used to mix by hand and had very inconsistent results, now we use a mixing attachment on a drill and it's the best. Perfect gunge every time, no lumps and consistent thickness.