I don't think there is an inexpensive solution to your problem. Lots of outdoor speakers available - look at outdoorspeakerdepot.com - nothing they have is selfpowered as far as I can tell. Would be really impractical to run both speaker cable and power all over a large backyard. Maybe even dangerous - I would not want to put 110v and amplifiers and power transformers out in the rain if I were a manufacturer. Centralize your power in a dry area - run lots of speaker cable.
I have 4 PAIRS of speakers in my backyard - and thinking of adding more (house lot is 11,000 sq ft - with big pool) - the issue is how to you get even coverage - consistent volume levels for a large area with no walls, lots of noise and listeners walking all over the place. I have a decent quality old 70's receiver hidden in my bar. I have Polk audio Atriums at ear level - then some rock speakers on the ground, then some cheapo full size box speakers (garage sale/thrift store) hidden in places to provide some bass. If these get weathered - I can replace them with another $20 pair. Forget audiophile sound outside - and settle for decent listenable sound levels in various listening spots.
My sound source is am/fm on the receiver and ipod line-out to the receiver. I would do airport express to an amp or receiver if that works for you and LOTS of speakers to provide coverage.
(permanent installation? what happened to tweaking?)