The six novels included in the Blackwater series were released in 1983. The next year, the National Library Service for the Blind and Disabled (a free program sponsored by the Library of Congress) released audio cassettes of the entire Blackwater series narrated by J. Michael McCullough. They were only made available through their network so it's not likely you'll ever come across one on eBay. I think you need a special cassette player for them anyway.
Luckily, the series has now been made digital and is available on their website. If you know anyone who qualifies for the free membership, this is a great way to introduce them to Michael McDowell's most beloved series.
Visit the National Library Service for the Blind and Disabled website for more details.