Such fasteners are usually made of soft silicone. If you can get them at a low cost it's just as well to go for those. Otherwise even a ball bearing fan should not vibrate a lot unless it's damaged, so it should be enough to use some simple fastening solution other than screwing them to the case. Anything that is less rigid and holds the fan in place securely enough, like ties, cord, strong tape or whatever you may find at home. Consider also that the fan might simply be going bad if it causes such vibrations.
Thanks! I will probably take a look at where I can buy a bunch of those fasteners for a reasonable price as that seems the best option (should eliminate any noticeable vibrations, and keep the fan secure). I don't think the fan is damaged - it's not a loud noise (unfortunately my PSU and hard drives drown out most of it), but I'd prefer to get rid of it