However, here's one you can quickly make yourself, with things that are very likely in your house already. The cost - zero. All you need to find is an empty milk jug, a push-pin, about a gallon of water, and a way to hang it up.
Fill a plastic, one-gallon jug with plain water. Put the jug's lid on loosely and poke a tiny hole (I use a push-pin) on the jug's side, near the bottom (about half an inch above the base). Hang the jug above a birdbath, or an old plate or tray, and let it drip. Tighten or loosen the lid (or poke holes in it) to adjust the flow. Refill as necessary.
This sort of setup is an efficient way to constantly water a plant too, although it may look to your neighbors like you're starting your own landfill.
Stick it to them. Lots of folks are fans of dragonflies and damselflies, in addition to birds. Here’s a simple trick to attract them; just stick a bare twig (2 or 3 ft. long) into the ground. In or near a wet or muddy area is best. Then just sit back and watch the “odonates” (as they’re called) perch on the end of it in hot, sunny weather, to watch for mosquitoes, which they eat voraciously.