Quite a few modern cases these days are coming equipped with 2.5" mounting with standard 3.5" HDD bays, Fractal Design cases are a good example of this. As others have mentioned depending on the SSD some come with tape or other means in order to secure the drive. You can always purchase additional adapters in order to accommodate the SSD, such as 5.25" bay adapters 3.5" bay adapters and some stand alone adapters, which are more difficult to find it seems.