You can read about IA here but TL;DR she’s a Vocaloid voice synthesizer program whose voice bank was donated by Lia.
Since just listing the Mekakucity Actors cover artists would be too simple, I’ll go the extra mile since I considered doing this before:
MARiA (from GARNiDELiA): Covered Children Record together with Jin and also sung the OP daze. Has also sung for shows like Kill la Kill and Mahouka. Used to go by the name Mai Mizuhashi where she sung themes for anime such as Kamisama Kazoku and Freezing.
Luna Haruna: Covered Kisaragi Attention. Popular Sony artist that Aniplex likes to force onto their big titles. Sung for shows like Fate/Zero, SAO and the Monogatari Series. Jin composed the Monogatari ED she sung which imo is the most unimpressive song he’s ever made, partly because she’s not a great singer to begin with.
Yasagure Koneko: Covered Mekakushi Code. I don’t know anything about her, but the song was arranged by I’ve Sound members so it sounds very distinctly like their work instead of Jin’s.
Shoichi Taguchi (from Sentimental Vector): Covered Kagerou Daze. He is both a vocalist and artist. He drew the Boku wa Tomodachi ga Sukunai+ manga and also illustrated for Sentimental Vector’s manga/music hyrbrid album Sheeloe, as well as their doujin works.
LiSA: Covered Headphone Actor and Yuukei Yesterday. Another Sony artist Aniplex likes to force upon their big titles. Sung for shows like Angel Beats! and SAO. Hugely overrated and also the only artist in this lineup with no known prior connection to Jin (as far as I know anyway). Wouldn’t be surprised if Aniplex pushed for her to be involved.
Kota Matsuyama (from BYEE the ROUND): Covered Lost Time Memory. Nothing to note other than that he’s the vocalist and guitarist of the band BYEE the ROUND. Don’t think he has any other anime connections either.
Aki Okui: Covered Ayano no Koufuku Riron. Sung for shows like Tasogare Otome x Amnesia and Turn A Gundam. Has a beautiful voice.
Takumi Yoshida (from Phatmans After School): Covered Kuusou Forest (easily the best cover next to Okui’s). If I’m not mistaken, he and Jin were classmates. Phatmans After School recently did the ED theme for Yozakura Quartet: Hana no Uta (Psst, watch Yozakura).
IA: ‘Covered’ Otsukimi Recital, which basically means it’s a new version of the song with a new arrangement. The Vocaloid in charge of singing all the Kagepro songs
after Miku was fired. I think they used her new voice bank for this? Not sure.
Jin: Covered Children Record (with MARiA) and Summertime Record. Creator of the Kagerou Project.
There’s still Konoha no Sekai Jijou unaccounted for since it’s exclusive to volume 7 of the anime. Don’t think the cover artist has been revealed but I’m guessing it’s Nanou since he sung it in the Live in Mekakucity concert.
As for downloads, well, you can find it at the usual places people go to pirate anime. Google is your friend.