I do not use injectable Estradiol, but I do know that any area with good blood circulation is a proper candidate for your injection sites. I recommend rotating to as many sites as possible to avoid eventually building up scar tissue from the constant needle punctures at just one or two sites.
Many people give themselves a sub q injection on their thigh. The large area and good blood circulation are a big advantage. Abdominal is good, but don't forget the upper arms in the deltoid area. If you don't have any problems turning at the waist, you can inject in the gluteus max.