I'm level 20 now (maybe 19, I forget) and I have yet to find a single Transfusion or Longbow grenade. I also didn't get a single corrosive weapon until AFTER I rescued Roland (thanks A LOT, game!).

I have, however, gotten at least half-a-dozen purple drops. I've heard of other people getting as far as I have and only seeing one purple. It's wild.

Also I <3 Tiny Tina. I want MOAR quests where I run around helping her blow stuff up, 'cause her commentary is FUNNY.