You can make your own enema syringe, but do be careful. You need to make sure that the part that is inserted won't hurt you, and you need to make sure that using it doesn't turn into a very unpleasant experience.
The water hydration pack sounds interesting, but for convenience and ease of cleaning I think I'd go for a Tupperware or Rubbermaid container. Get one of about a pint to a quart. A quart will let you do two or three fills before refilling, but you probably shouldn't go bigger unless you're interested in really deep play. The more water you put in the more you disturb your system and the longer it takes to come back out.
From the hardware store plumbing department I'd get a threaded brass fitting to push-on barb adapter as well as the tubing. If you have a rounded, smooth enema tip you can get any type, but if you don't I'd go for a soft tube made of the amber latex or, if you can get it, silicone (but soft is important here - you don't want to put a hole in your rectum). Drill a hole in the Tupperware, screw the barb adapter in, put the tube on, then either put your enema nozzle on the end or round the edges of the "butt end" of the tube so you don't scratch or irritate your anus/rectum.