Who are the known experts in forehead injections (to correct a flat spot, not to fill wrinkles)?

It sound like you are trying to reshape your entire forehead. It's difficult to give a precise answer without a picture. Depending on how much volume you need, it is possible that temporary fillers may be able to help, but you may require large volumes that make the cost similar to possible surgical options, and it may not actually achieve your desired result. Depending on severity, options will range from temporary fillers, fat grafting, custom implants, surgical burring/contouring of the frontal bone, and even cranial vault remodeling. 
Many different provides can talk to you about fillers, and maybe even fat grafting, but for the other options, i would seek out a Plastic Surgeon with craniofacial experience. 

Thanks for sharing your question. Dermal fillers are a good treatment option for volume loss in the forehead. Usually, the filler needs to be combined with Botox/Dysport for the best result. My preferred filler for this indication is Restylane although Restylane Lyft and Juvederm Voluma work as well. So injectors prefer to use a microcannula. The treatment is safe and effective in experienced hands. There are several great injectors in the New York area, several of whom are active on this site.