My Preview kept crashing so I uninstalled it. Now trying to reinstall but it’s not available in the usual places. Please help! Thanks

Your welcome.

Are you referring to Preview or some third party app called "My Preview". If you are referring to Preview, then you cannot uninstall it as it is considered part of Mac OS. However if for some reason you cannot access it, restart in the Recovery Partition (Command + R on startup) and then re-install Mac OS. This will not erase your data however it will fill in the parts of Mac OS that are missing. That being said, ALWAYS have a complete backup of your system before doing an install.