Oregon RFID only sells HDX and FDX-B products which operate at 134.2 kHz, the international standard frequency for animal identification.
FDX-A operates at 125 kHz. There is no HDX that operates at that frequency. This is primarily used for security and entry systems (parking lot gates, hotel room doors) but some glass tags are available.
Most readers will detect either one frequency or the other but few read both.