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".
And there lies my technology fall back I have never had a MP3 player and have no computers or anything but the cell phone.... Not that I'm not willing to learn but I'm afraid I'm going to open up yet another can of worms....
