Joe’s Computer Museum, an online store (and more!) operated by Apple II enthusiast Joe Strosnider, now has the LiteSound Stereo Audio Interface card in stock. This Apple IIGS expansion card by Bob Darlington adds stereo sound output, which the IIGS inbuilt audio jack does not natively support.
The card costs US$42.50 plus tax and shipping and includes an audio cable; a second cable can be added for another $5.75. At the time of this writing, 22 cards are currently in stock.