Here is a source for pre-made Polyurethane envelopes and motors. I use the 1/3 oz motors on my 6 ft blimp.
http://www.airtimekite.com/ sells bladders for kites.
Their Orange bladder material is an EVOH/LDPE laminate - very tough and light weight, and much easier to heat seal than PU. For making a long thin blimp - a hyperblimp - you could do worse than buying the biggest leading edge bladder they have (about 24 inches circumference) and working from there.