Where to get the bootcamp software for bootcamp Windows 10 iMac late 2013 21.5 inch

Hello there, triasadi.

It sounds like you are looking for the BootCamp Assistant Software on your Mac. When running OS X, Boot Camp Assistant is available from your Utilities folder per the information found in the following Knowledge Base article:

How to install Windows using Boot Camp

Open Boot Camp Assistant

Boot Camp Assistant guides you through installing Windows on your Mac. Open this app from the Utilities folder in the Applications folder and follow the onscreen instructions to automatically repartition your startup disk and download related software drivers for Windows. If you're prompted during installation, connect a blank USB flash drive.

If in fact it is not located in your Utilities folder, since it is a System Application you will need to reinstall OS X software by following the instructions for OS X Recovery in this article:

How to reinstall OS X on your Mac

Reinstall OS X from Recovery

If have a recent Mac, you can use OS X Recovery to reinstall OS X.

1. Turn on your Mac.
2. Immediately after you hear the startup sound, hold the Command and R keys on your keyboard.
3. Release the keys after the Apple logo appears.
4. If prompted, select a Wi-Fi network, or connect your Mac to the Internet using an Ethernet cable.
5. After the Recovery menu appears, select the option to Install OS X.
6. Follow the onscreen prompts to select your destination disk and install OS X.

These steps reinstall the latest version of OS X that you previously had installed on your Mac. If you want to reinstall the version of OS X that came with your Mac, hold Command-Option-R at startup instead. This starts your Mac from Internet Recovery.

Thanks for reaching out to Apple Support Communities.

Cheers.

Can you use BC Assistant to download W10 drivers package? Check the Product Version line in Bootcamp.xml. If it is a 5.x version, it will not work on W10.