Hi, Flax! Besides the sites recommended by Cathynap, you can consult:
Books of entries from 1923:
Dirección Nacional de Migraciones
Hipólito Yrigoyen 952 - Capital Federal (Ciudad Autónoma de Buenos Aires)
Tel.: 54 11 4342 9531 / 54 11 4317 0272
E-mail:
info@migraciones.gov.ar / Web:
http://www.mininterior.gov.ar/migraciones/horarios.asp
Books of entries from 1882 to 1929:
CEMLA (Centro de Estudios Migratorios Latinoamericanos)
Av. Independencia 20 - (1099) Capital Federal
Tel.: 54 11 4342 6749 / 54 11 4334 7717
E-mail:
base@cemla.com / Web:
www.cemla.com
At CEMLA website, you look for a surname and, if you find it, you ask for more information, which has a cost. (I know it's $ 12 for Argentinians, but I don't know how much it costs for foreigners.)
Good luck!