For seltzer water - co2 tank + regulator + "diy carbonator caps" + recycled 1L or 2L seltzer bottles
You should be able to buy a co2 tank and reg on craigslist from someone liquidating their homebrewing equipment for pretty cheap.
You don't need a regulator.
My setup is:
Paint ball Co2 tanks; 2 20oz tanks One was used / craigslist 2nd was from Amazon
ASA valve - Ebay
small pressure gauge - Ebay
1/4" nylon reinforced tubing, 1ft - hardware store
"carbonator caps" stainless steel 2 they screw onto 2L soda bottles - Amazon
ball lock; a quick disconnect use on beer and soda kegs, fits the carbonator caps - Amazon
1/4" fittings; barbed for connecting tubing, T - hardware store
Use:
1.8L of filtered water into a 2L soda bottle ('head' room for the Co2) put on a carbonator cap
Connect the Co2 to the bottle via the ball lock.
Slowly open the asa valve, watching the gauge. Turn off the valve at (about) 60 psi. Disconnect the ball lock.
I lay the bottle on it's side for more surface area
You will probably want to 'top off' the Co2 back to 60 a 2nd time - the pressure goes down as the Co2 is absorbed by the water - I don't like my soda very bubbly
Also - I get my Co2 for cheap by buying dry ice, crushing it in a tyvek envelope with a piece of 2x4 and funneling it into my tanks. I use a digital scale to control how much I crush / put in. I also weight the bottle w/ dry ice before putting the valve back on.
Note: you need a way to clamp the tank and a BIG wrench to get the valve off the tank the first time. I used a 3.5" muffler clamp bolted to a board.