where do I find osx installer

Both the Instructions and the Links for the OS are here...How to create a bootable installer for macOS - Apple Support...Click the link for the one you want and you will be directed to the App store for Download.

Before Downloading read the Compatibility list to make sure your choice matches your computer.

Also read carefully the info for creating the bootable installer.

Good Luck...🍀

<<Mid 2015 15" MacBook Pro. I need an external startup disk to clean up my hard drive. >>

No, you don't. You can use Internet recovery, and it will load everything it needs from the internet, including Disk Utility.

Hold Command + Option + R at startup.

what model Mac?

if after 2013, Internet Recovery (includes Disk Utility) is in your Mac's ROM, and can be booted up even if the drive has been removed, provided you have an internet connection.

Hold command Option R at startup.

https://apps.apple.com/us/app/macos-catalina/id1466841314?ls=1&mt=12

Use another mac computer

So using an External drive is good advice if you do not have a backup.

That External drive must show up in Disk Utility, and needs to be ERASED and made into a MacOS drive. Then you do the Install (you can use recovery) onto that newly-created External Volume. Defaults of GUID partition Map and MacOS extended Volume are usually appropriate, unless you are already using Catalina.

If and only if you are willing to walk away from the originals of all your old files, you can:

boot into Internet recovery

erase the entire drive, using the defaults for Catalina which will be GUID partition Map and APFS Volume structure,

then do the Install (Recovery can be used for this)

I did not suggest you install, I suggested you use Disk Utility.

Did you use Disk Utility?

were you able to repair your disk?