First, casting lots is not the same thing as divination. Divination is one of the forbidden practices mentioned in the Old Testament, and it involved the attempt to foretell future events or discover hidden knowledge by occult or supernatural means. Diviners were just one class of people forbidden to practice their "arts" in Israel's midst. Others included wizards, questioners of the dead, witches, sorcerers, people who purge children with fire, augurers, omen readers, and enchanters (though this is not an exhaustive list).
Clearly, the early 11 apostles were not seeking God's guidance by occult, magical, or supernatural-apart-from-God means. Notice they
"prayed, 'Lord, you know the hearts of all. Show us which one of these two you have chosen to assume the task of this service and apostleship from which Judas turned aside to go to his own place'" (vv.24,25).
The apostles knew enough to seek God's face first. This is significant, I feel. They also considered carefully the qualifications of the two "applicants" (see vv.21-23), much as the early church in Jerusalem did when they appointed deacons (Acts 6:1-7). Finally, they did not "put it to a vote," with 51 percent of the vote carrying the day. No, they prayed and then they put out a "fleece," so to speak (see Gideon's experience in Judges 6:39,40). In short, their hearts were in the right place.
More in answer to your question: Yes, the apostles were promised the baptism of the Holy Spirit. Moreover, Jesus gave them a "temporary" endowment of the Holy Spirit to "tide them over," so to speak, prior to Pentecost (see John 20:22,23).
I have two questions: 1) Did the disciples truly know and appreciate fully what Jesus did in John 20 when He breathed on them and told them to receive the Holy Spirit?; and 2) Did they appreciate what being baptized with the Holy Spirit signified (see John 15:5-15 and Acts 1:8)? To both questions I have to answer no--at least not fully.
As with so many of the things Jesus told them (sometimes over and over again), Jesus' words went right over their heads. It was not until after His resurrection that they "remembered" His words and were able to make sense of them (see Lk 24:8; Acts 11:16).
The apostles did at the time the best they knew how. They did not know (which we do today with 20/20 hindsight) that Jesus' "temporary" endowment was probably sufficient for making this important decision, and that they therefore did not need to resort to an Old Testament method such as the Urim and the Thummim to confirm God's will in the matter (see Ex 28:30; Lev 8:8; Ezra 2:63; and Neh 7:65).
Perhaps some even better questions (and I'm surprised you did not ask them) would be:
Why did the apostles feel it was incumbent on them to appoint a twelfth apostle?
Did Jesus reveal His will to them in this matter prior to His death and resurrection, and His words simply did not find their way into the canon of Scripture?
Furthermore, were the eleven being presumptuous in concluding that a twelfth apostle was indeed God's will for them at that time?
Had God really wanted Paul, who had yet to appear on the scene, to be the twelfth apostle (though I do not personally feel this was the case), and by electing the twelfth apostle themselves they "cheated" Paul of the honor?
Before I close, I'll share a personal anecdote you might find enlightening and constructive. I had a professor in Bible College who told us of a desire he once had to join with the Wycliffe Bible translators in their translation work among people-groups having no Bible in their heart language. The day came when his application papers were all ready to be signed and then mailed to the mission's headquarters.
Of course he had prayed about the decision, seeking God's face and will, and he by no means took the decision lightly. When he reached the dotted line at the bottom of the application form and was just about to sign his name, something unusual happened: he forgot his name! Yes, he forgot his name completely, and no amount of wracking his brain would bring it to mind so that he could sign the application. He took his mental block as a "sign" God did not want him to become a Wycliffe Bible translator.
As with Gideon's fleece, the Urim and the Thummim, the occasional casting of lots, and perhaps any other non-biblical way of making an important decision, my professor cautioned us that this type of guidance should be the exception to the rule and not the rule itself.
In conclusion, I have found that prayer and fasting (individually and corporately); the advice, opinions, and confirmation of godly people; and last but not least the witness of the Spirit within us (especially the gift of His peace when making an important decision), are more than sufficient to determine God's will regarding the majority of decisions for which God's Word does not provide clear answers.
When you've exhausted those three avenues of guidance, however, and you still do not have a clear answer, I say pursue other means of guidance (e.g., a "fleece"), and then expect an answer, and possibly a miracle!