There are a ton of backing tracks to be found online, but most are not specifically aimed at harmonica players. To redress the balance, I have published three volumes of Blues Harmonica Playalongs as book/CD packages, each available in English and German:
Steve Baker: Blues Harmonica Playalongs vols. 1 - 3, artist ahead Musikverlag
These are instrumental tunes in a range of blues and related styles, recorded with a great band. Each song comes in two versions on the CD, one with my harp and the other as a playback without harp. The book contains transcriptions and detailed descriptions of what I played and how I played it. Available from the publisher here: