Are there any substitutes for squash puree?
If you want something storebought, just get pumpkin puree - it is squash puree, not really even a substitute. Pumpkins are squash, and "pumpkin" is a pretty broad term, covering a lot of winter squash. In fact canned pumpkin is often made with things more similar to butternut squash than what you'd think of as pumpkin (jack-o'-lanterns and all). Just make sure you don't accidentally get some kind of pumpkin pie mix that has spices and maybe other things added.

I suppose if you can't find that either, you could also look at baby food, since squash is fairly common there.

If you really can't find anything in the store, though, don't just give up on the recipe. It really is incredibly easy to make - just cut squash in half, throw in the oven cut side down, bake until it's soft, scoop it out and mash up a bit. The actual active working time is only a couple minutes, and you'll probably get much better flavor than you would from canned goods.