Last night I ordered my alternative 3M ATG tape from http://findtape.com. The item is named
JVCC ATG-7502 ATG Tape (Acid Neutral) (that also means acid free). I ordered the 1/4" x 36 yds @ $1.98 each. I am really happy the website tells you the 3M brand name you can replace.
Oh! If you have the ATG 700 and want to use the 1/4" tape, you can purchase an adapter. I am not sure where I purchased mine from since I purchased it right after I purchased my gun back in 2010. Just shop around because some places charge WAY more than others.