You can fill them any way you wish. Little Red Caps are pretty good low pressure bladders. They are available for about $5 for 20 in a pack. Here we get them from Lee Valley Tools. I think Ace hardware in the US can get them ordered. They are used for caulking guns and work well for that too. They degrade after a few weeks when fuel is on them, and after a year on a caulking gun depending on conditions. You could try Texas Timers for standard latex tubing. I found the local medical supply had some well priced tubing in the 1/16", 3/16" and 1/4" size. A thin wall is preferred if you can find it. The TSC store has some 1/4" stuff that works ok for .15 size stuff.