Problem downloading stock ROM :(

Hello everyone I need serious help.I was trying to root my Samsung galaxy s3 T-Mobile(t999) and I did some mistake and got stuck on samsung boot logo.I searched for stock ROM for my s3 but i'm not able to get it from its official sites.I tried to download from sammobile.com but couldn't.so I optd for samsung-updates.com and I found stock ROM but every time the download about to finish it stops and says retrying and retrying even though I have a strong network cntn I tried to download from everywhere but m having same problem.to ensure if m getting this problem from my WiFi.so I switch to my 4g data connection unfortunately same happened anybody plz help me download firmware for my s3...