See
Forcing for IZF in sheaf toposes
Also
Toposes from Forcing for Intuitionistic ZF with Atoms.
Edit:
Maybe more references:
Heyting-valued models for intuitionistic set theory
The book "Intuitionistic logic, model theory and forcing" by Fitting.
see also the following web-page where some references are also given:
Constructive Set Theory: Forcing, Large Sets, and Mathematics