Actually, there are several convenient places to find St. Augustine online!
There’s the huge document storage website Documenta Catholica Omnia, which has recently changed its URL. It’s now documentacatholicaomnia.eu/ Most of this stuff is from Migne’s Patrologia Latina or Patrologia Graeca series, although occasionally they have documents from other public domain sources.
Another good source is Monumenta (monumenta.ch/). It’s a Swiss scholarly site giving the text of various Fathers and medieval/Renaissance writers in Latin, with links to various digitized manuscripts of those works.
But for Augustine specifically, the best site is Augustinus.it, run by some monks over in Italy that are publishing new editions of Augustine. The handy thing is that like Monumenta, they have the texts in a digitized form that you can easily search or run through translation sites. Obviously Google Translate isn’t as good as knowing Latin, but it’s not bad. (And sometimes Google already has a translation on file for a specific text, so magically you see something better than you expected.) Augustinus.it has versions of the site in various languages. So try the English site first. If that’s not showing what you want, try the Italian site, since it’s updated faster.
The site lists two books similar to what you want under “Latin Works”.
One of them is “Questionum in Heptateuchum libri septem.” That’s the one you asked for. You want “Liber Sextus.” (That’s Book Six, or literally, the Sixth Book.) It’s called “Questiones in Iesum Nave”, which means “Questions about [the Book of] Joshua son of Nun.” (Yeah, “Nave” is what happens when Hebrew is transliterated to Greek and then transliterated to Latin.) It’s a bit longish, but you might be able to get some hints by running it through Google Translate.
The other one, which you may find of interest, is “Locutionum in Heptateuchum libri septem.” When you get to that, you go over to the side and click on “Liber Sextus.” Book Six is called “Locutiones de Iesu Nave,” which means “Phrases in Joshua son of Nun.”
It’s a pretty short volume, and Augustine is commenting directly on various verses. So you should be able to get a pretty good guess from Google Translate. It may answer questions for you that the Questiones didn’t.
Hope this helps!