There is a lot of carries for garrosh in my server recently and a lot of challenges groups trying to sell me a carry but If I ask for a gold challenge group they already have a mage, buuuuu
they deny you a carry? or just a group in general?
frost is strong as shit in challenge modes, one of the best specs you could bring.
i would suggest running with some guildies, preferably people who haven't done it before.
the official forums have a pretty great cross realm CM thread as well, vert active.
its filled with people willing to take on newcomers to CMs (if your spec is appropriate that is).
i will never find a group for my shadow priest unless i heal, which i don't much like doing lol... would have to pull some favors.
the last time i did CMs was on my main, and that was 1 week after we finished t14 progress... so that would be around 2/5/2013
back then we didn't have bis lists etc, we just used what we had and specced appropriately, its much easier now, especially with all the guides etc.