Sure you can go buy water guns from Target, super soakers are NOT cheap though. OR you can save your dish soap bottles! Seriously, they work great as 'water shooters'. They are also the perfect size for small hands to grip & squeeze. It's simple: save your empty dish soap bottle, rinse well, remove the label (optional), fill w/ water, make sure the cap/spout is on, then SQUEEZE! You'll get a great stream of water AND save some plastic from the land fill.
To save water & your sanity when kids are running in & out to refill the bottles, just place a bucket of water outside for *refills*.
OF COURSE, it goes w/o saying to use extra supervision w/ little ones around bucket of water: they tend to be top heavy & can flip into a bucket in an instant.
As an alternative to 'water bombs', you know those round, cloth balls or football shapes, sold by the water toys? Well, you can purchase giant sized sponges at any $.99 cents store or Dollar Tree and have the kids toss those around. These also make great replacements for water balloons. Water balloons leave a bunch of tiny bits of rubber/latex all over your lawn--not fun & not good for your lawn or birds that might pick them up. Forget the mess and toss a soaking wet sponge back and forth instead! This way you also have a product you can reuse around the house when everyone is done 'playing'. Again, keep a bucket of water *outside* for instant 'refills'.
If you're feeling really crafty you can sew a nifty cloth cover over your sponge, like the brand name 'water bombs' have...OR....check out these super crafty & adorable 'wet water sponges' (aka: DIY water bombs).
Aren't these the cutest water toys you've ever seen? Don't you want to run out *right now* & buy some fresh sponges to make these? ME TOO!
One last tip for outside fun with water...a friend said she sends her kidlets outside w/ a small bucket of water & paint brushes to 'paint' the sidewalk. Great! Call it secret, invisible spy art, when it dries & you're set!