The only way to get them is from battle. Just keep fighting and fighting and your units should get more. Retraining averages the current chevrons with the chevrons of the new recruits. If the unit is almost full and you retrain it, you won't lose a whole lot but if the unit only has a few units left you could lose quite a few chevrons. Merging units does the same thing but you can choose which units to merge. You can merge two units with the same amount of chevrons and you won't lose any.