2.5" SSD in a 3.5" desktop drive bay normal?

I would just buy a 2.5" drive and use a mounting kit:

Now that they all use the same SATA ports, you just use the same standard connectors as any other hard drive.

Most SSDs tend to fit a 2.5" form factor, although there are some exceptions such as the OCZ Colossus. Adapters/brackets/mounting kits can be bought from most online computer stores. A Toshiba/Kingston SSD is one of the few drives that comes with a 3.5" adapter, but I wouldn't recommend the drive in terms of performance. I can't advise on the pros and cons of the different adaptors, as personally my SSD just "rests" in a 3.5" slot - there are no moving parts so I'm not that concerned about it.