Lets say I want to compose a little 12 bar blues from loops. Now if its in the key of C major, I should be OK

C / / / (position 1) . . .   F /// (position 4)  . . .   C /// ...  G /// (postion 5) and so on . . .

But if i want the composition to be in, say, A major, the choice of A doesn't seem to be available just A minor (Ab?) or Bb major