We are not familiar with any commercial suppliers of membrane form IgG, but we'd suggest that you contact some of the suppliers who specialize in providing immunoglobulins. Here is a starter list:
ATHENS RESEARCH & TECHNOLOGY INC.
BIOLEGEND
IMMUNOREAGENTS INC.
JACKSON IMMUNORESEARCH LABS INC.
PROTOS IMMUNORESEARCH
ROCKLAND IMMUNOCHEMICALS INC.
SOUTHERNBIOTECH
THE BINDING SITE IMMUNOLOGICALS GROUP
You can find their contact info. here: https://www.linscottsdirectory.com/suppliers