Strange or funny but I can find tubes in that size at the shop I helped at this summer (things have slowed so I'm back on the customer side of the counter). They even have had at times a 26"x1-3/8 tube.

Have you determined where the flats are on the tube? Are they in the same place each flat? Are they doubled/pinch/snakebite flats? I've found a few problem tires where the only way I could find the problem with recurring flats was to inflate the tube out of the tire until the hole revealed itself, then used that to find the approximate location on the tire, then bending the tire casing in that area basically inside out where I sometimes find little almost microscopic wires sticking through the tread and puncturing the tube slowly over time. I've also found little bits of embedded glass shards this way too, gotta be carefull not to cut yourself while looking and feeling for the sharp stuff though.