I am making a recipe that calls for Roasted Barley. LHBS didn't carry. I have some leftover Flaked Barley. Can this be roasted in oven to make roasted barley?
No. In a pinch, you might get away with roasting 2-row, but flaked won't work. Roasted barley is produced from unmalted whole barley (so, even roasted 2-row wouldn't technically be correct).
I'm surprised that they didn't have Roasted Barley. What are you making? If it's just a little for color you can swap it out. In small amounts you could substitute Black Patent, Chocolate, Carafa, or Carafa Special.
I am making a Bass Ale Clone (Biermuncher). I have seen other recipes w/o roasted barley. I ordered from Austin Homebrew. I could only find dark roasted barley.
Flacked barley is unmalted barley that has gone through a gelatinization process to release the starches. So, it really won't give you the same results as starting with raw barley. Also, the difference between roasted & totally burned is very small.