Didn't go back the post but-
They used to make 1/2 speed cassette decks specifically for that.
I'd be tempted to "just" use a computer and a freeware program (like Audacity) and "half speed" mp3 files.
Zero cost.
half speed (in case you didn't know, again without reading the "tascam" post) makes everything exactly one octave lower, allowing you to keep the guitar tuned "standard".