The Drumming System has some excellent double bass exercises for beginner, intermediate, and advanced players.
For beginners, the key is to play consistent patterns with your feet at different note values (eighth notes, eighth note triplets, sixteenth notes, sixteenth note triplets, and thirty second notes). It’s important that you practice along with a metronome.
Alignment and accuracy are key here. Speed isn’t.
Intermediate players should play exercises that break up the beat a little bit with different note values. Advanced players should practice shuffles and more complex patterns. In addition, advanced players should work on patterns with broken sixteenth notes and broken thirty second notes. At this level, increasing speed is important and fun.
One double bass exercise that will help you develop control fast was given to me by my teacher, Tobias Ralph. Simply play alternating sixteenth notes with your feet (R L R L). At same time, play sixteenth note double strokes on the snare drum (R R L L). This actually forces you to keep your feet even. If you played alternating sixteenth notes on the snare drum instead of double strokes, it can allow you to let the pattern slip into a sloppy shuffle. Keeping this even is important.
The Encyclopedia of Double Bass Drumming by Bobby Rondinelli and Michael Lauren contains a lifetime supply of material. It has tons of grooves, fills, solos, a variety of styles, and a variety of time signatures.
I’ve found that one of the greatest double kick exercises is also one of the greatest exercises for overall fitness: BICYCLE RIDING! If you think about it, the motions used while riding a bicycle are strikingly similar to playing double bass. So get on your bike and ride!