Originally Posted by SkyDog75

Tubes are sized by wheel diameter and tire width. You have 700c wheels, which have a bead seat diameter of 622 mm. Your tires are nominally 38 mm wide. So you need 700c tubes whose rated range of widths includes 38 mm. The third thing you need to know is what type of valve to look for. If your original tubes use a valve just like the ones you'd find on your car (which is likely since the bikes are hybrids), those are called schrader valves.

So putting it all together, something like this:
XLC Boxed Tube, 700 x 35 - 43C (27 x 1-3/8), SCHRADER