It's a separate part, I would be surprised if you couldn't find it separately somewhere
https://www.ifixit.com/Guide/Nintendo+DSi+Touchscreen+Replacement/3751
I'd probably look on ebay or aliexpress for parts, I couldn't tell you if any sources are preferred or not
https://www.ifixit.com/Guide/Nintendo+DSi+Touchscreen+Replacement/3751
I'd probably look on ebay or aliexpress for parts, I couldn't tell you if any sources are preferred or not