Just to make sure: Agents are of static level. Agent that offers level 1 missions will never offer level 2 ones, you'll have to find an agent that does.
Level 1 agents are always available to you.
Level 2 agents become available at 1.0 standing.
Level 3 needs 3.0 standing.
Level 4 needs 5.0.
When doing missions you get rewarded with standing for the corporation and the agent on standard missions and faction standing on storyline missions that you get every 16 normal missions.
To access the agent you will need enough standing with any one or more of the following:
The agent you are trying to access (eg. Okaseilen Fukashi). This will let you access the agent even if your standings drop below the required level for other options.
The corporation you are doing the missions for (eg. Caldari navy). This will let you access all the agents below what you have, so if you have that 2.9 you'd be able to access all level 2 missions within that corporation.
The faction (eg. Caldari state). All NPC corporations belong to some faction and if you have enough standing you can access any corporation that belongs to that faction. So if you had 2.9 standing with caldari, you'd be able to access, say, Home guard level 2 missions without having to grind through level 1s first.
And finally Eve-agents
might be of some use when looking for agents to do missions for.