reinstall mac os x v10.5 on a 2009 IMAC

Good morning all, my sister recently gave me an imac she bought back in 2009 she asked me to go ahead and wipe it which i did (hopefully the right way) but when trying to reinstall the Mac os on it it says file cannot be found in the app store, thats when i try and log in with my apple id, could anyone point me in the right direction how i could download a copy of the OS and put it on a usb drive and try and reinstall that way? im fairly new to iMacs so any help would be greatly appreciated, i have been reading for hours online and have tried everything i have found and no luck.

thanks everyone! have a great day!

Luis