Where do I find a download for Garageband 10.10.5?

Apple has two policies that make your situation difficult:

• Only the latest version of an app is available for download. You don't have access to earlier versions and because the App Store is the only source where you can legally get those apps, you are out of luck.
• Apple is not big on backward compatibility. The current version requires a recent macOS version to run. For example, GarageBand and Logic Pro X require at least macOS 10.11 (El Capitan).

GarageBand on the Mac App Store

Hope that helps

Edgar Rothermich - LogicProGEM.com

(Author of the "Graphically Enhanced Manuals")

http://DingDingMusic.com/Manuals/

'I may receive some form of compensation, financial or otherwise, from my recommendation or link.'

I accidentally deleted it by mistake and want to download it again.

Are you signed into the Mac AppStore with the AppleID you used to purchase GarageBand 10? Then try, if you can download the last compatible version for your system again from the fourth tab of the AppStore "Purchases". Sometimes it works to reinstall an older version you already purchased. Edgar Rothermich's pessimistic summary is unfortunately correct, but sometimes the AppStore offers to download the last compatible version for your system, if you cannot run the current version. Just try it.

Welcome to the Apple Support Communities!

I understand you deleted the GarageBand application from your iMac. GarageBand is not included in the operating system on the Mac. You will find your copy either in the Purchases tab of the Mac App store, or by restoring the file from Time Machine. I'll include some resources below to help you with this.
Redownload apps, music, movies, TV shows, and books from the iTunes Store, iBooks Store, and App Store
How to use Time Machine to back up or restore your Mac​
macOS Sierra: Recover items using Time Machine and Spotlight​
Take care.