Fallout 3

I think its on a person or raider near by but not for sure

Actually I believe you get that key from Three Dog. When you visit Rivet City and talk to Doctor Li before competing Three Dog's quest.

If you visit Rivet City or come across a certain vault you will trigger the next quest, essentially short circuiting Three Dog. If you then go to GNR studios Three Dog will ask you to retrieve his dish. As you already know more about Dad he'll offer you a bonus to get you to do the quest. That is the key to the special cell in Hamilton's Hideaway. That's the only way to get it.

Theres 2 ways to get the key 1 from 3 dog 2 from raiders near by i found a key on a raider he had a gun called "fat man"

*spoilers*

Before Completion of main quest "Galaxy News Radio", you need to learn of your father's journey to Rivet City, either by convincing Three Dog to just tell you, or by travelling to Rivet City beforehand. In an effort to give you something for your trouble, Three Dog will give you the key as a quest reward.

The key is only obtainable as a reward from Three Dog at the GNR building in the D.C. Ruins. In order to get the key you must first accept to do him a favor by fixing the dish at the top of the memorial monolith thing. Then after fixing the dish BEFORE going to see him either visit Rivet City and speak to Dr. Li or visit Vault 112 and enter tranquility Lane to obtain the info Three Dog would hav previously given you on your dad as a reward. He will then reward you instead with 'Three Dog's Cache Key' which will allow you into the room in Hamilton's hideaway. This is as far as i know, the only way to obtain the key and i learnt this from the Official Collector's Edition Strategy Guide so i'm 100% certain it's correct.