Yeah, digging on the wiki is probably the best way to do it, and is definitely not illegal. (Providing you don't do anything illegal with the sprite once downloaded).
Alternatively, you can use
Game Launcher to download older versions of Terraria, then use
TExtract to extract the files. (Both tools are hosted on the forums so therefore are not illegal/against Re-Logic's wishes). The latter version would require you to sort through item ids, which could take a while, so I would only recommend it if the wiki does not contain the files you seek.