The oem's won't come with brackets to fit the 2.5 ssd into the 3.5 bay right?
That is correct. The beauty with the SSD's though is that they are so light that you can put them anywhere in your case, in any direction you want.
I have used double sided sticky tape on my SSD and it is sitting in the middle of an empty 3.5 slot. It is virtually unmovable. Just make sure you give your SSD a tiny amount of airflow, just in case...
I have seen people mounting them on the top of their case upside down, in the space between the top DVD-ROM and the case, etc, etc.