When you buy a SSD they will sometimes come with an adaptor to put the SSD in a 3.5" bay. On my caes the 3.5' bays had one slot that had a built in adaptor and in the front it had two hot swap bays that could hold a SSD without using an adaptor.
SAMSUNG 830 Series MZ-7PC128D/AM 2.5" 128GB SATA III MLC Internal Solid State Drive (SSD) Desktop Upgrade Kit
As an example this SSD from Samsung come in kit form and that means it includes the adaptor for a 3.5" bay. So if looking for a SSD then try to get one that comes as a kit and you'll get the adaptor.
With SSD's being more popular case manufacturers are making builtin adaptors for the SSD.