Here we have an obvious typo in the 1879 LDS edition (“that I might spy our their forces”). Even so, this reading was copied twice into two subsequent LDS editions, the 1906 large-print edition and the 1907 vest-pocket edition. There is only one other occurrence in the text of the verb spy and this too has the adverbial out: “to spy out in what part of the city the Lamanites did camp with their army” (Alma 62:20). The replacement of out with our here in Mosiah 9:1 may have been influenced by the use of our in the following clause (“that our army might come upon them and destroy them”).
Summary: Maintain the original use of the phrasal verb “to spy out” in Mosiah 9:1.