Terraform AWS AZ error - manually specify AZ?
If anyone else has this issue, here's how I got around it.

Specify a variable with the availability zones you want to target

variable "zones" {
  default = ["us-east-1b", "us-east-1c", "us-east-1d"]
}


When creating your subnets, use this line

availability_zone = "${var.zones[count.index]}"