Wearing corsets over Lolita might be tricky because your corset generally needs to be short and not restrict the bust or the poof of a petticoat, so as not to interfere with the traditional Lolita silhouette.
Some brands release corset-like tops that are lightly or un-boned and not meant for waist cinching. Things are referred to as bustiers.
The Japanese brand Excentrique is most well known for their steel-boned corsets and corset dresses. Not necessarily a Lolita brand, Excentrique follows a similar aesthetic as Lolita.