This is a guide on how to get infinite stimpaks, radaway, .556 ammo and ♥♥♥ grenades in Fallout 3
First you need to go to Fort Independence which is located west of Fairfax ruins and north of Andale.
There you will meet the Brotherhood Outcasts.
Talk to Defender Morgan about joining and she will propose you to collect tech for them.
Now go get a couple of Laser Rifles (Laser pistols, power armor pieces or plasma weapons work good too).
Once you gathered some tech for them go back and give it to Protector Casdin who is right outside of the Fort Independence main doors, inside the fenced area.
Once you have given all the loot to him just make a quick save and crouch.
Try to steal all the items back and if you fail just load the quick save.
When youre succesfull in stealing an item just make another quick save and keep going until you have all the items back.
Then just trade the items back to him and keep stealing them off of him once you got all of them back.
You can also trade him Scrap metal, sensor modules, alien blasters, alien power cells, enclave power armor and tesla armor according to the wiki but I havent tried any of those because laser rifles are pretty easy to get. If you dont have any laser rifles just go to the Fort Bannisher and kill some Talon guys to get laser rifles.