First off, I have been playing roms for over a year.
As long as the site you download it from is a special rom site, it should be safe, or if the youtubers have a link to download it in the description, I would use that too.
My favorite site is coolrom.com which contains countless Pokemon roms.
Theflamingspade is a very popular youtuber who gives safe links to download roms. The ORIGINAL download link and thread for Pokemon liquid crystal is here and is safe and risk free, so download it here if you want to play the game: http://www.pokecommunity.com/showthread.php?t=242023
As for legal purposes, don't worry about it. Nothing bad will happen as long as you are playing and downloading it on the computer. It is only illegal if someone sells it. Because these download links are free, nintendo won't come to your house and sue you are anything like that. It is completely safe.
Please read before you start downloading
I am assuming this is your first time downloading and playing a rom. To play a rom, you have to download visual boy advance. The safe and risk free link to download it is here: http://coolrom.com/emulators/gba/14/Visual_Boy_Advance.php
Once downloaded, open it and click "load gameboy game". Select the rom you downloaded and press okay.
Source: My own experience.