You should not require mounting brackets for 3.5" HDDs or if you do in your particular case, they should have been included in your case's hardware bag/box.
For 2.5" HDDs and SSDs, some cases have trays that can accommodate both 3.5" and 2.5" while others have dedicated places to mount 2.5" drives. If brackets are required, they are, again, usually in the case's hardware bag/box.
Unless your case lacks mounting spots that match whatever drive size you want to put in it, you should not require aftermarket brackets.