I've just invested in an Ultrabreathe which I received yesterday. I initially chose it because I was looking for a cheaper alternative to the Powerbreathe but reading the write-ups it seem it is more flexible with the settings It is adjustable so you can increase the workload as and when. It strengthens muscles while inhaling and can also be used to do the same for exhaling.
I'll use it and let you all know the outcome after week one, week two, etc.They say improvements should be seen after about a month. I got mine from a divers online shop.