Everyone is experiencing the same problem. I have had to check for days to get one suitable aircraft. Its because the game is so full. If you have a route you need to jump on right now, go ahead and get one aircraft from another manufacturer with the same capacity or close. Having a common fleet is important, but the way things are in this game, fleet commonality is not a high priority for me like it use to be. And it really isn't going to hurt you to have a couple of other types in your fleet. Fleet commonality is in the top 3 things I try to do, but expansion is number one at this point. I have had mixed fleets before and nothing terrible happened. I might have saved some money, but it worked out ok. I just fool around with the fares and raise them as often as I can (maybe 1-2% per day depending how conditions are) without losing my load factors and still make money. I'm pretty sneaky about it when comes to fares. Otherwise, its all a waiting game if you are sticking to fleet commonality. And its a long wait as other players will agree.