FO3's level system was designed and balanced for twenty levels. The game world is huge but there's no reason you need to do everything with one character, it has excellent replayability.
As many people were unhappy this Bethesda did released a DLC that raised the cap to 30 and continued the main quest called Broken Steel. However since they weren't able to re-balance the system and gameplay leveling past 30 (even with the tougher monsters) is rather unbalanced and even a low INT character can max out all of most of their skills without trying, and obviously that removes a lot of what makes characters unique. However the main quest extension is quite nice.
I'm not terribly familiar with the PS marketplace but I believe Broken Steel can be purchased individually. The GOTY edition also comes bundled with all of the DLC together.
User-made mods are PC exclusive, neither MS nor Sony allows them, so I've moved your thread out of the mod forum and in to FO3 General.