The first one that you can find that is able to be found early in the game in the RCPD. When you go through the West Wing after getting the Combat Knife, you can go up a set of stairs above the West Office, the same set of stairs that you need to go up in order to obtain the Spade Key. On 2F when climbing the stairs, the only room that you can enter is the right-most side of the Shower Room. As soon as you enter, look right, there will be a portable safe sitting on some towels. While I am unsure if this randomizes per playthrough, here's how I solved it. I mapped out a grid to get a "number" combination.
The "dial":
---4-6---
--5---8--
--2---7--
---1-3---
The buttons:
1-2
3-4
5-6
7-8