When you have nothing to mix paint with, you grab scrap. Unless you really, really need to do some serious mixing. Then you improvise.
One of my final tasks in my shop build was to paint the concrete walls. I’d already bought paint, but after a couple months of sitting around it had fully separated, with several inches of clear liquid on top of the settled pigment. I have some paint stirrers somewhere amid the myriad boxes I have yet to unpack.
I could have used a piece of scrap, but this needed a lot of mixing and I didn’t feel like stirring half the afternoon. Now, I also have a couple mixer things you can use with a drill, but they’re also in a box somewhere. Easier to find, however, was a short length of dowel. And, after drywalling all the framing for the shop I had buckets of drywall screws close at hand. This was the result:
Not pretty, but once chucked up in my drill it did the job of mixing the paint very nicely. And, rinsed off with the hose I could even save it for future use. Of course, once I put it in a box, I’ll never find it again.