I tried party goat slime but I thought it was kinda gritty and didn't really love it. You might want to try it though it wasn't that expensive. Messy supplies was great though
I ordered from messysupplies.com and it was good. Best suggestion is if you have a friend, combine orders to cut on shipping costs, but I got it in like 4 business days to the US.
I'm in the UK myself, where we're well provided for! But I know a couple of US friends who are making their gunge with Tylose powder, from catering/bakery suppliers. It's CarboxyMethylCellulose, so it seems to be at least somewhat in the ballpark of those other cellulose compound thickeners. They add a small amount of J-Lube (or equivalents like K-Lube or X-Lube) too, to give it a little more stretchy/stringy consistency.