A while ago there used to be a tool called CheckFUS you could use to download Samsung firmware, however that no longer works.

So the usual option was to use a site like SamMobile or Samsung-Updates to download the firmware, the problem is the server are often SLOW.

So I stumbled across this tool on XDA called SamFirm, It downloads the files direct from Samsung meaning you get them alot faster.

To use it, Simply enter your model number (mine is GT-I9505) and your carrier code (TNZ, VNZ or NZC), tick Auto and click check update. It will go and check, and after a minute or so the download option will light up on the right hand side. Tick Check CRC32 and Decrypt Automatically, then click download. Once it's finished doing its thing, You will be left with a Zip file, inside that Zip file is the tar.md5 file you flash with odin!