There are some bladders that are being used that look like a kitten sized condom.  They are used for resealing caulking tubes and are available at Lee Valley Tools in Canada for about $5 for 20 of them. They are a very thin wall, low pressure balloon thickness pretty much like a pacifier. They don't last too long but are ok for a couple of sessions likely. A normal needle could be used because of the lower pressure.  I just bought some latex tubing from the medical supply store that is surgical tubing with a 1/16" hole and a 1/8" OD that I plan to use for a bladder.  We used to use a cigar aluminum case or a suitable sized syringe without the plunger that was trimmed and screwed on to the side of the plane's profile fuselage. It could be stuffed into a combat likely too, or just attached to the top of the wing somewhere handy.