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What sort of parts do you have access to? If you can list the PVC parts you have access to, that would be best. Anything else you think might be good could also help. Let me know so I can tell you what you can do if the other options I mention don't sound good.
PVC is available most everywhere from what I know, so you should have access to at least enough parts to make something good. A few other people from third-world countries have had success finding a good PVC stock, as reported to me by email. PVC would make the best water gun for this purpose, but there are other options.
Homemade water guns can easily be made from two liter bottles, and there's loads of ways you can accomplish that. Some people have tried stupid things like putting balloons inside of two liters bottles... that is a bad idea and I would not recommend it. There's a basic two plastic bottle homemade water gun on our website:
http://www.sscentral.org/tech/homemades/2l.php
Please note that I did not use a two liter bottle. Also, the tee (shaped like the letter T) used is completely optional. I used that mainly out of personal preference.
You shouldn't have any trouble duplicating what is shown in the pictures. Charging was done with a simple hose attachment. Let me know if you have any other questions on that water gun if you decide to go the way of the bottle.
Piston water gun also are easy to create and obtain. Look for a few water guns that operate through pistons at any toy stores you have access to. You also could make a simple piston water gun with just PVC pipe and endcaps, the most simple water gun around.