This option is well known and popular in the enthusiast community, which is quickly adopting SSDs and discovering that most desktop cases weren't designed for 2.5" disks. The idea is simple: basically you’d go for 2.5” SAS hard drives and install them into an existing 3.5” environment using available metal mounting frames. These can be obtained for very little money, and they attach on both sides of the hard drives in an effort to make the construction exactly as wide as a 3.5” drive.
However, while this approach works well in pedestal PCs and servers, where hard drives are installed into and integrated with the entire system, you will run into trouble once you try to do this in drive bays designed specifically for 3.5” drives and their easy replacement. The critical item here is the position of the SAS connectors. Mounting a 2.5” disk with installation frames usually results in dislocated SAS connectors. As these will most likely end up at a different position as it were the case with true 3.5” drives, you will probably not be able to actually connect your drives to the ports inside your drive cages.
This solution works great electrically and costs very little, but it is not applicable in mainstream server scenarios.
Can you see the different location of the SAS connectors? The aluminim frame around the drive has the connectors at the right position for 3.5” drives.
Electrically, any 2.5” drive works just fine in an installation frame, but mechanically you might end up with displaced connectors.