It’s hard for you to look for the original Air-Byte IR transceiver nowadays, and it’s a better choice to DIY yourself.
Most of the parts used here are not critical as you’re thought, such as the NPN darlington transistor driving the infrared LEDs, the PNP transistor used for inverting the IR detector output and the similar materials can, easily replace all other components.
For this project, the main brain of the Air-Byte transceiver is a tiny 8-pin PIC12F508 microcontroller, which the operation is very simple. The PIC is used for monitors GPIO,0 connected to the PC serial port TX pin #3. This also means, the 12F508 will generate the 40kHz IR carrier, if only the pin transitions from logic low to logic high, otherwise, it is suppressed.
Thus, it provides a simple method for receiving the serial data on one pin, and then output the serial data on another modulated at the IR detectors. This makes the band-pass frequency very simple and effective as well.
Enjoys the moment and have a nice DIY day!