There are dozens of shôjo manga magazines, and every year new ones appear, while others disappear. I have listed shôjo manga magazines and ladies' comic magazines separately, but keep in mind that a few magazines do not really fit into the category they are assigned by their publishers. For example, Bouquet and Petit Flower are defined as shôjo manga magazines, but in practice they have really been ladies' comic magazines for several years.
Also keep in mind that shôjo manga magazines are not the only manga magazines girls/women are reading. Among the 500+/- girls I surveyed at a high school in Kobe in October '94, Weekly Boys' Jump was far and away the most popular manga magazine among girls, as well as boys. The second most popular manga magazine among girls was Special Edition Margaret, with about half as many mentions. And so on.
The figures for the top ten in each category come from the 1998 edition of the Shuppan shihyou nenpyou ("Annual Report of Publishing Indicators"), are are therefore probably pretty accurate. They reflect circulations in 1997. The figures for the remaining magazines in both categories come from the 1997 edition of the Zasshi-shinbun sou-katarogu ("Complete Magazine/Newspaper Catalog"), published by the Media Research Center, Tokyo. These are claimed circulations, and are much more likely to be inflated (which is why some boast circulations higher than some of the magazines in the top ten list!).
Most of the top-sellers should be available in any store—such as the Kinokuniya shops in Manhattan and Edgewater, NJ—that carries a considerable number of Japanese periodicals. If they aren't available in a Japanese bookstore near you...complain!
The information is given in the following order: the title transliterated into the Latin alphabet; an English translation of the title in parentheses; the period of publication; the publisher; the circulation; the readership (according to the publisher). My own comments (if any) follow.
Although I include links to the web sites of publishers and individual magazines, only Shogakukan provides pages in English, and none of the magazines do.
Ribon ("Ribbon");
monthly; Shueisha;
1,630,000; 60% primary-school girls, 30% middle-school girls. Home of YOSHIZUMI
Wataru's Marmalade Boy and SAKURA Momoko's Chibi Maruko-chan. 
Nakayoshi ("Good
Friends"); monthly; Kodansha;
780,000; 65% primary-school girls, 35% middle-school girls. Home of TAKEUCHI
Naoko's Sailor Moon and CLAMP's RayEarth. 
Bessatsu
Maagaretto ("Special Edition Margaret"); monthly; Shueisha;
730,000; 80% middle- and high-school girls, 15% college women and older.
Home of TADA Kaoru's Itazura na kiss and such artists as IKUEMI
Ryoh, HIJIRI Chiaki, FUJIMOTO Mari, and TSUMUGI Taku. 
Za
Maagaretto ("The Margaret"); monthly; Shueisha;
400,000; 9.3% primary-school girls, 29.4% middle-school girls, 30.5% high-school
girls. 
Bessatsu
furendo ("Special Edition Friend"); monthly; Kodansha;
260,000; 6% primary-school girls, 33% middle-school girls, 40% high-school
girls, 21% college women and older. 
Bessatsu
shôjo komikku ("Special Edition Girls' Comic");
monthly; Shogakukan;
200,000; middle- and high-school girls. 
Shôjo
komikku ("Girls' Comic"); monthly; Shogakukan;
440,000; primary-, middle- and high-school girls. 
Misuterii
bonita ("Mystery Bonita"); bimonthly; Akita;
300,000; primary-, middle-, and high-school girls. 
Purinsesu
goorudo ("Princess GOLD"); bimonthly; Akita;
300,000; primary-, middle- and high-school girls. 
Derakkusu
maagaretto ("Deluxe Margaret"); bimonthly; Shueisha;
330,000; 75% middle- and high-school girls, 20% college women and "office
ladies." 
Bessatsu
hana to yume ("Special Edition Flowers and Dreams"); monthly; Hakusensha;
200,000; girls and women of middle-school age or older. 
Jurietto ("Juliet");
bimonthly; Kodansha;
220,000; middle- and high-school girls. Also known as Bessatsu Furendo
DX ("Special Edition Friend Deluxe") 
Buuke ("Bouquet");
monthly; Shueisha;
150,000; 12.3% middle-school girls, 27.3% high-school girls, 57.8% college
women, "office ladies," and housewives. 
Yuu ("You");
monthly; Shueisha;
500,000; 45% 21-25 year-old women, 29% 26-31 year-old women. Still the top-selling
ladies' comic magazine. Fairly conservative, but of generally high quality. 
Komikku
Amuuru ("Comic Amour"); monthly; San Shuppan; 360,000; "office
ladies" and housewives in their twenties and thirties. Unapologetically
pornographic manga by and for women. Not for the weak of heart. 
Bii
rabu ("Be-Love"); monthly; Kodansha;
350,000; 58% "office ladies," 34% housewives, 8% college women. 
Chao;
monthly; 
Maagaretto ("Margaret");
biweekly; 
Hana
to yume ("Flowers and Dreams"); biweekly; 
Rara ("LaLa");
monthly; 
Purinsesu ("Princess");
monthly; 
Asuka;
monthly; 
Uingusu ("Wings");
monthly; 
Puchi
komikku ("Petit Comic"); monthly; 
Puchi
furawaa ("Petit Flower"); bimonthly; 
Judi ("Judy");
monthly;