Get a variety! At least if you hope to use knots away from home, like boating, climbing or camping. I’ve definitely found it useful to practice knots on different sizes, materials, tensions, etc., just because the ropes I get in the wild vary… for camping you might be using 1/4 inch or smaller, for a small sailboat maybe 1/2 or 3/4-inch, when I go on a houseboat every once in a while, it’s like maybe 1.5 or 2-inches diameter, sometimes old and fraying, sometimes nylon and relatively slippery, or sometimes a natural fiber rope that’s much grabbier especially when wet…

Knots really seem to behave differently on different sizes & kinds of ropes, and it can be hard to remember (for me at least) how to tie a knot you learned with twine on a bigger boat line. How easily they bind, how well they hold, and how easily they release also starts to get way more important when you’re outside and depending on the lines.

You can buy kits from proknot.com The one I have comes with two small lengths of cord and two small carabiners (I'm a climber).

My only complaint is that the cord tends to be a bit stiff but that's unavoidable if you're using real cord.

https://proknot.com/products/pro-knot-knot-tying-kit