I have a tire 26 X 1.4 what size tube should I buy for it? 1.25 or 1.5 Inner tube ?
Most 26x1.25 sized tubes are rated from 1.25" up to 1.5". You should be fine with the 26x1.25 tube as long as that's the lowest size indication on the tube.
Though most tubes you'll find will indicate an acceptable low/high range. You're likely to find the tubes indicated as 26x1.25/1.5 which should fit any sizes listed or between.
Are you saying you can buy 1.5 - 1.75 now but they don't have 1.25 - 1.5 in stock?
If so yes 1.4 is very close to 1.5. As with any install be careful not to pinch the tube when installing the tire. Fill it up to 10 psi and release pressure to relieve folds / twist.