You generally don't need much drop for bullhorns as they often have a section that drops below the stem. Pretty much any road stem with some drop will work for you.

If you need a stem >120mm you may want to find a bigger frame. You may find a long quill stem and bullhorns to be too flexy. But a lot depends on the stem, bullhorns and rider, and it sounds like you're not having too much trouble with your current set-up.

Where are you located? If you're in the US, universalcycles.com, carries the Nitto stems in 130 (as do others) and seen them in a few shops such as Freewheel, SF.